Economically Refuel Your Spacestation (How?)

[Riph]

I'm feeling pretty ambivalent toward mods at this point. I could open up liquidEngine1/part.cfg and give the LV-T30 a billion thrust and a zillion isp right now, and it would solve all my engineering problems, but I don't think anyone would agree that this is a good solution. Obviously the more popular mods make more effort than this to be balanced, but balance is always going to be a judgement call on the part of the mod author.

That said, I want to thank einsteiner for being the second person to actually include a .craft file with his post. Us newbies want to learn from your designs, oldfolks. Post those craft files!

[Wanderfound]

After flying this thing so much, I began to realize that I know almost nothing about the 'correct' way to get a mostly air breathing craft into orbit.

With spaceplanes, the general idea is to maximise both your horizontal speed and altitude before lighting the oxidising fireworks. Basically, get into the thin, low-drag air as fast as you can (20,000m; you may want to keep at lower altitudes until you're over Mach 1 to avoid transsonic nose-tuck problems, though) before flattening out to build as much horizontal speed as possible (aim for at least 1000 m/s; get it just right and you can do better than 2,000m/s) before the engines choke at around 30,000m. Once they do, pull the nose up to 45 degrees, light the fuse and go to space.

There are assorted tricks to extend how long the air lasts. Keep your climb rate and angle of attack as low as possible (by 30,000m they should be both be below ten), shut down engines in pairs rather than all at once, throttle down a bit once they start to gasp, etc.

The problem with doing this with non-spaceplane SSTOs is that they usually have lousy horizontal flight capabilities. Just like with planes, if your CoL isn't behind your CoM then the thing is going to try and flip backwards when you level off.

[Riph]

There are assorted tricks to extend how long the air lasts. Keep your climb rate and angle of attack as low as possible (by 30,000m they should be both be below ten), shut down engines in pairs rather than all at once, throttle down a bit once they start to gasp, etc.

I was already doing most of these, and am delighted to hear them confirmed by an experienced spaceplaner. Most of all, I'm grateful for the actual numbers in your post (1000m/s, then 45 degree climb) as they're actual, measurable goals that I can use to improve my flight plan.

[Wanderfound]

Another trick: don't shut down the air breathers as soon as you activate the rockets. Air pressure is a function of both altitude and speed; when you light the fireworks, the acceleration should increase the effective pressure on your intakes and kick the jets back into life.

Do shut the intakes if the jets die, though; they create a huge amount of drag.

[Riph]

don't shut down the air breathers as soon as you activate the rockets.

This strategy screwed me again and again when the left or right engine would flame out, resulting in there now being enough oxygen to run the opposite engine, resulting in a wild horizontal spin.

Finally I realized that all this mess could be avoided by having a single centrally placed turbojet, which obviously cannot send me into spins when it flames out. Every single one of my SSTOs since has had a centrally mounted turbojet, which I basically just leave on forever. The ramjet effect of the rocket engines causes it to be awake well into 50km and higher, though by then the thrust it provides seems negligible. (Thrust seems to scale down as available oxygen drops?)

[Kerbart]

For all of those who posted so far: don't get a job in sales (which really, is not what most of you want in the first place).

The problem isn't "cost". The problem is "yield" which is revenue - cost. You can increase yield (or reduce negative yield) in two ways:

• lower cost
  
• increase revenue
  

While lowering cost is always a good idea, the much simpler solution is to increase revenue. Have some experiments tag a long the ride and let Kerbodyne, Flooyd Dynamics and Jeb's Cannery (or whatever) pay for your fuel runs! A less optimal design that is fairly tolerant to the ascent profile and costs $30,000 but can bring in $15,000 in revenue will be cheaper to use than a $20,000 design that is so critical that you cannot attach any experiments to it.

[Wanderfound]

This strategy screwed me again and again when the left or right engine would flame out, resulting in there now being enough oxygen to run the opposite engine, resulting in a wild horizontal spin.

Finally I realized that all this mess could be avoided by having a single centrally placed turbojet, which obviously cannot send me into spins when it flames out. Every single one of my SSTOs since has had a centrally mounted turbojet, which I basically just leave on forever. The ramjet effect of the rocket engines causes it to be awake well into 50km and higher, though by then the thrust it provides seems negligible. (Thrust seems to scale down as available oxygen drops?)

Yup, same trick here. Get to 30,000, shut off everything except the central turbo, hello Mach 5. The only reason the D7 has a pair of turbos on the centre line was that the tuning-fork layout of the design made a central engine impossible (but it has enough inertia that a brief asymmetric thrust near the centreline is no big deal). This is also why I usually switch modes manually on the RAPIERs; if you wait for the auto-shift, they may not go at quite the same time.

[Wanderfound]

For all of those who posted so far: don't get a job in sales (which really, is not what most of you want in the first place).

The problem isn't "cost". The problem is "yield" which is revenue - cost. You can increase yield (or reduce negative yield) in two ways:

• lower cost
  
• increase revenue

Ain't nothing stopping you from hooking a part test or two to your SSTO, and that's what I usually do. The fuel bill of around √5,000 is usually massively outweighed by the profits.

[Laie]

(on keeping jets running long past their due time):

This strategy screwed me again and again when the left or right engine would flame out, resulting in there now being enough oxygen to run the opposite engine, resulting in a wild horizontal spin.

If you take the time to place intakes & engines in just the right order, this may help a lot.

Kasuhas thread about fuel flow rules (air flow comes at page 5 or 6): http://forum.kerbalspaceprogram.com/threads/64362-Fuel-Flow-Rules-%280-23-5%29

A short explainer in another thread: http://forum.kerbalspaceprogram.com/threads/84217-Explaining-burnout-asymetry

If you make it so that all engines will be equally short on air, you'll find that you can keep them running for quite some time after the resource panel shows that intake air has dropped to zero. Flameouts will eventually happen, but a) at much higher altitude, so you get to utilize your jets for longer, you will get several seconds worth of audible and visiual cues to prepare you for the event. If c) you ignore these cues and get a flameout, it will be easier to recover from your mistake.

[Riph]

(From the second thread Laie linked)

When doing intake air calculations, the program runs through the .craft description in order; when it hits an intake it adds its air to the IntakeAir pool, when it hits an air-breathing engine it subtracts its requirements from the pool. Since parts are added to the .craft description in order of placement, it matters what order you place the parts on the ship.

In my example of the two engine, eight intake design, the way most of us would put it together is to place all the intakes at once, then place all the engines. So the .craft description would be (simplified, of course):

Intake

Intake

Intake

Intake

Intake

Intake

Intake

Intake

Engine

Engine

So the first engine finds the IntakeAir pool filled by eight intakes and takes its full requirement; the second engine is left with whatever the first doesn't take, so it will starve for air first. This produces asymmetrical flameout.

If instead you place half the intakes, then an engine, then the rest of the intakes, then the other engine, the .craft description looks like this:

Intake

Intake

Intake

Intake

Engine

Intake

Intake

Intake

Intake

Engine

So when the program runs the IntakeAir calculation it gets to the first engine with four intake's worth of air, which the engine then takes. The next four intakes fill the pool again for the second engine to clear. This is the recipe for symmetrical flameout.

To put this knowledge to work: Build your ship by placing half the intakes, then an engine, then the remaining intakes, then the other engine. Alternately, build an assembly with one engine and half the intakes and duplicate it with symmetry. Never place engines alone with symmetry.

This is not very intuitive or obvious, kudos to Kasuha for figuring it out.

Holy crap this is a lifesaver. I'm going to go rebuild my planes right the eff now.

[RuBisCO]

Care to suggest a specific craft, Rubisco? My spaceplanes get themselves into orbit neatly and cheaply, but they can just barely get a 1 ton satellite into orbit.

If it can't get a rockomax 32 (at minimum) into orbit in one trip, then really you're only saving kerbal currency at the expense of player time.

Your going to want bigger spaceplane parts then stock. Try spaceplane plus for example.

[Mister Spock]

Can I ask a newbish question? Once you do get that orange tank in space, how do you dock it to your space station? Does your tug dock with it, then haul it to the space station? Is the tug basically an LV-N with a small fuel tank and a docking clamp?

I don't mean to derail a useful thread, so feel free to ignore me; I'm just so newbish that I struggle to refuel at all, never mind refueling economically. I have three half-empty orange tank "space stations" around Kerbin, and another two around Minmus. I don't know an efficient way to join them all up!

[Godot]

The most important thing is the docking clamp on the orange tank (and another one on the station).

After you docked the tugand tank basically become part of the station ... that means, if you want to reuse the tug (instead of keeping it attached to the tank, which also is an option)

you also have to have docking clamps between tug and orange tank.

In this case you just undock the tug from the tank in order for it to become an independent spaceship again.

Also, the tug of course also has to have a probe core ... not just engine, tank and docking clamp ... and enough batteries and solar cells to keep the probe core powered

[Traches]

Ooh! I just had an idea!

Why can't your sooper heavy SSTO spaceplane be your station too? Put it in orbit, refill other ships till it's nearly empty, then land it and launch another. Saves you the time of rendezvous and docking, and you don't even need RCS.

[LordFjord]

Can I ask a newbish question? Once you do get that orange tank in space, how do you dock it to your space station? Does your tug dock with it, then haul it to the space station? Is the tug basically an LV-N with a small fuel tank and a docking clamp?

I don't mean to derail a useful thread, so feel free to ignore me; I'm just so newbish that I struggle to refuel at all, never mind refueling economically. I have three half-empty orange tank "space stations" around Kerbin, and another two around Minmus. I don't know an efficient way to join them all up!

Either the tug docks it, or the delivery craft, or the receiving vessel.

Here's an example of my heavy SSTO (0.23.5) that hauls 4 rockomax x200-32 tanks (72 tons of cargo) into Kerbin's orbit. The biest is ~250 tons on the runway and has >600 parts.

The cockpit section is detachable - serves as an escape vehicle and also as a tug with a claw to move the cargo around. This way the big SSTO does not need RCS on its own.

Mind you, it took me a long time to get there - building and flying SSTOs of this size - wouldn't have figured it out without many of the folks around here.

I haven't attempted to build a cargo SSTO in 0.24 yet, but soon it will be due. I expect it to ruin me from the testflight and fails.

As for refueling stations as cheap as possible with rockets, use as many SRBs as possible. Might try that as well, could be a fast and cheap way to get fuel up there - without the effort of SSTOs.

You might want to take a look at the vessels in this challenge: http://forum.kerbalspaceprogram.com/threads/87211-0-24-s-Cost-Effective-Lifters-Challenge

I think that is exactly what you want

[Tuareg]

(From the second thread Laie linked)

Holy crap this is a lifesaver. I'm going to go rebuild my planes right the eff now.

lol, that is so lame, typical squad

[Foxster]

The problem with the SSTO plane solution is that they take so long to do their thing. Takes a while to get them up and a while to get them back in one piece. In terms of time, I find the best refuel solution is just a plain old rocket lifter that I recover some of.

About 75K funds will put an orange tank into 85K orbit, with just enough fuel left to de-orbit the top of the lifter and kick up some dust on landing under some 'shutes. The "secret" to economic lifters is to use SRBs for the first two stages where you'd probably use engines+fuel in sandbox, with a small engine stage for finishing the orbit off.

Once you have a man on Mun and Minmus plus a probe in orbit around each you can make enough off flag and orbit science contracts to shove tanks into space.

Edited July 30, 2014 by Foxster

[Wanderfound]

Can I ask a newbish question? Once you do get that orange tank in space, how do you dock it to your space station? Does your tug dock with it, then haul it to the space station? Is the tug basically an LV-N with a small fuel tank and a docking clamp?

I don't mean to derail a useful thread, so feel free to ignore me; I'm just so newbish that I struggle to refuel at all, never mind refueling economically. I have three half-empty orange tank "space stations" around Kerbin, and another two around Minmus. I don't know an efficient way to join them all up!

The way I usually do it is to put a probe core, a solar panel, a docking port, a roundified monoprop tank and a few RCS thrusters on the orange tank itself. Detach tank from spaceplane, dock tank to space station, done.

Alternately, dock the spaceplane to the station, pump the fuel across and then carry the empty tank back to KSC. It all depends on whether the station just needs fuel or whether it could use some extra tank space as well.

[Javster]

Do you use debrefund? You can recover much more parts with it.

[Riph]

Can I ask a newbish question? Once you do get that orange tank in space, how do you dock it to your space station? Does your tug dock with it, then haul it to the space station? Is the tug basically an LV-N with a small fuel tank and a docking clamp?!

You have it exactly right, Mister Spock. The orange tank has a docking clamp (I like the Sr model) on each side, such that one side gets docked to my nuke tug, and the other can be driven straight into an open port on the space station.

Here is a picture of one such tug from a prior conversation: http://cloud-4.steampowered.com/ugc/580151430942899249/6D9B1A47E4AC80D0705CAB7306B8A84E3BAA5672/

Now that funds are out, the four LV-Ns on that model are a bit excessive. But, because this thing will orbit and work for you forever, it will pay for itself in time. You could use fewer engines for more efficiency, but you'd have boring slow burns, and boring is not the kerbal solution.

Also note the giant SAS wheel on the tug. Fuel tanks are heavy, and you'll need lots of torque from reaction wheels or RCS if you want to move them effectively.

[Mister Spock]

Thanks for answering my questions, everyone. My first launch window for Duna is approaching, and I've been practicing by setting up refueling stations above Kerbin and its moons. This thread will help.

[Pecan]

Thanks for answering my questions, everyone. My first launch window for Duna is approaching, and I've been practicing by setting up refueling stations above Kerbin and its moons. This thread will help.

Check chapters 7 and 8 of my tutorial (link in signature) for some designs to try.

@ Riph - I like yours too :-)

[Mesklin]

Any chance we can get a .craft file for the Hexagon lifter, Mesklin? I'm trying to build something similar and I can't squeeze half the performance out of it that you seem to have.

No problem, sir, here is subassembly with this craft. Place it in "Subassemblies" folder in your save.

[SpeedDaemon]

One thing you can do is put any spare fuel on the station. Make it back from an interplanetary trip? Don't land with any fuel on board - dock and put it on the station first. Same thing if you find you've over-built an ascent stage. Put the spare fuel on the station instead of hauling it all the way wherever else you're going.

If you're not opposed to mods, get Karbonite or Kethane and build a mining and refining infrastructure. Minmus is best. This way you're basically tipping fuel tanks down into a gravity well, and sending the empties back up instead of the other way around. The catch is that you'll probably have to spend 200-300k or more on miners, converters and stations (and you have to be good at landing in very specific spots to be most efficient with mining).