Delivering fresh fuel to an orbiting tanker - need a cost efficient method

[wossname]

Say we have a tanker in LKO (100,000m circular). It's empty now since it expended all it's go-juice getting up there in the first place.

I want to fill it up again by sending up several smaller vessels with fuel to replenish the tanker. This sounds like an expensive thing to do if done naively.

Does anyone have any suggestions about the most *cost* effective way to re-fill a tanker in LKO?

Lets assume that the tanker has 5 large orange tanks on board and we want to fill them to at least 80% capacity. Supply ships may be salvaged for re-use where feasible.

Would it be most cost efficient to do many launches of a small supply ship or one or two trips of a big supply ship? I suspect the answer to this isn't simple but I would like to hear your ideas on this.

Does anyone have a handy "rule of thumb" for this sort of thing?

[Gaarst]

It depends on the rockets you use to do it.

You can always make several types of rocket and calculate the cost per ton of fuel to the station for each type, which will tell you which way is the cheapest.

[SDEngineer]

Post 1.0, the most cost effective way to refuel anything is probably to build a fuel refinery on Minmus (or in Minmus orbit, with ore ferried by a mining ship from Minmus itself). Fairly cheap to set up the infrastructure, assuming you have the appropriate tech nodes unlocked (maybe a half million to set up a fuel depot in orbit, a miner/ore carrier for the ore, and a fuel tanker to move fuel wherever you need it). Drawback of course would be the time to set it up and the time it takes to mine and move the ore - but the dV savings over time is frankly enormous. You only spend a few hundred dV to get the ore (and therefore fuel) to the Minmus fuel depot, and then (assuming you aerobrake properly) another few hundred dV for the fuel transfer vehicle to get to LKO to actually do the refuel, and aside from the initial launches to put those craft in orbit, no further wastage as those craft will never need to return to Kerbin. Essentially at that point, it's free fuel forever for anything that has made it to LKO. Given that you're looking at 5 orange tanks, the savings will be huge, and it's likely cheaper to set up a Minmus fuelling operation than to find a way to launch 5 full orange tanks worth of fuel into orbit (my 40t launches to LKO, which is roughly equivalent to 1 orange tank, are typically well over 100K/launch).

However, if you're dead set on lifting that much mass from Kerbin, you're right, it's not an easy answer. What you're looking for is the payload that you can lift to LKO expressed as tons, then divided by the cost of the total spacecraft to get a kerbal kredit/ton cost. While I haven't done any math on this, simply based on my own designs I think the "cheapest" way to do it is with a ton of relatively small launches - I can get 5t into orbit at well under 10% of the cost for a 40t launch, even though my payload fraction for a 40t launch to LKO is definitely higher.

Edited May 12, 2015 by SDEngineer

[Randazzo]

If you're sending up fuel from Kerbin, the answer relies entirely on your designs. I would lean towards as few launches as possible, but if your rocket is only twice the size and triple the cost, that's obviously not cost effective.

A fuel refinery and infrastructure requires a significant inital expenditure to get everything in place, and afterwards it's really just exchanging the monetary cost with a time investment to refine and deliver the fuel. If you do go that route, you'll need a delivery vehicle capable of delivering it's entire load and returning to it's station without refueling (or using any of it's payload fuel) for maximum efficiency.

Edited May 12, 2015 by Randazzo

[Laie]

Does anyone have any suggestions about the most *cost* effective way to re-fill a tanker in LKO?

Would it be most cost efficient to do many launches of a small supply ship or one or two trips of a big supply ship?

Post 1.0, the most cost effective way to refuel anything is probably [resource mining]

This. Mining can't be beat.

Other than that, the larger engines and tanks tend to be more weight-efficient. If that also makes them more cost efficient is something I haven't tried yet. Probably your best bet -after mining- is to try yourself at vessels where most if not all parts reach orbit and can be landed back at the space center. The Quad Cluster and Liquid Fuel Booster are notable for being quite resilient, making them easy to recover.

Need not be a strict SSTO: throwing away a few SRBs may be more cost-efficient in the end.

There's also the question how much your time is worth and what kind of stuff you enjoy. Getting it over with in as few launches as possible may be a valid design goal in and of itself.

[wossname]

Thanks for the tips guys. My career mode game hasn't yet advanced to the mining stage yet but it sounds like a great project to build a fuel factory on Minmus! I'm definitely going to try that.

[TheXRuler]

If you like using expendable rockets you should try using many many SRB's (at least in the first stage). I am a fan of far too huge and entirely too expensive interplanetary motherships with dedicated landers etc etc or ridiculous 1200 ton space stations. Most of the time I use SSTO spaceplanes to get my stuff up (It's a lot cheaper since you only pay for fuel) and if you can be bothered then that (or a minmus refinery) is definetly the way to go. However spaceplanes have the big problem that their cargo bays are only ever so large mk3 in stock or s2 wide in b9 aerospace (.90). For larger payloads I use a launcher made of a 9 core solid rocket first stage, if you want my construction techniwue just pm me I'll explain it. And then a poodle upper stage, manages something like 45 tons in .90 with oldFAR installed, haven't tested it yet. The thing is, SRBs are bloody cheap "trash cans full of boom" as somebody here fittingly said, and while they are quite heavy for the dV they provide and have a low ISP it's still worth it from a cost standpoint

[KerikBalm]

Do you have turboramjets?

Delivered over half an orange tank to Munar-orbit... about an orange tanks' worth of oxidizer, but just about 200 units over half an orange tank (an X200-32), got back with 60 untis of LF

Likewise, that was a cargo-carrier that delivered a munar lander and addon battery+ solar array to the station.

-The station had just been refueled by the other SSTO... when getting there, the 2nd stage with the skipper was empty, but I decided not to stage it, so I'd have more fuel capacity at the station, which should be especially needed when I put ISRU operations on Mun (I did the same thing with a very similar station to Minmus... the Mun has some ISRU advantages that Minmus lacks, so I'm doing both)

[vicwarrior]

The best would be to set up a mining ops in either minimus or the mun (personally i like more minimus), have mining rig there mine for ore which you can have refined either on the ground, or in orbit (doesn't matter since ore to LF+O is mass neutral).

Once you have enough fuel up in your fuel depot you can have a tug (i recomend nukes for it) pull it to your refueling station in kerbin LKO.

It would be a big investment for this set up but you would save much more spacebucks in the future by making interplanetary missions much cheaper as a result of being able to refuel for free at your space station.

[n0xiety]

With the new mining system everything is too easy. I don't even bother with refueling stations anymore. Just built myself a SStM (Single Stage to Minmus) mothership that carries about 20 kerbals and fuel refinary with 8 drills on it. It takes off from Kerbin and then lands at Minmus for refueling. After that its easy to go anywhere with the spare 6000dv. Ship itself costs about 550k and im playing career too. 950t full weight. Well you can always use something like this to carry fuel from Minmus if you still insist on refueling stations.

[bmyers831]

I do this pretty well by doing these together-

1) I don't build 'minimum' boosters for each launch. Build a set of maximum boosters instead (max 1.25, max 2.5, max 3.75 etc). Use the one that makes sense for your primary payload and drop off extra fuel at the tanker- helps keep it topped up for very little extra funds. Get good at landing boosters for huge savings (this is where you really make it big). By maximum I don't mean adding twenty radially attached solid boosters- just build a good solid core stage. A lot of my launches use 2.5m boosters which are just a core SSTO and can launch anywhere from 5-20 tons all the way to a circularized orbit and still have enough fuel to land back at the KSC. If I'm just tossing a small satellite or two up into orbit I can usually add a 10-ton fuel tank to it's payload and deliver that fuel to the tanker/station.

2) bring back fuel from a low gravity environment such as Mun or Minmus. For me, I use a dedicated orbital transfer vehicle (this is what needs fuel) and a cargo lander for Mun and Minmus. These bring up spare fuel for the transfer vehicle that was gathered on site just like I do from Kerbin with the boosters.

3) if fueling from Kerbin, consider launching a fueling mission directly to the ship needing to be fueled (in a kerbin parking orbit). You save any fuel you would have spent doing the tanker rendezvous twice (once from the vessel needing fuel and once from the incoming fuel delivery).

4) ships that spend their time in space moving between kerbin and wherever should all be nuclear powered for fuel efficiency. They go a loong way on very little.

I think my last .90 mission put two satellites into orbit which fulfilled a total of 6 missions and a fuel tank for my station. The satellites earned over 100k per contract and the booster landed back at the KSC. My profit was something like 600k, and I delivered a lot of fuel to a thirsty ship at the station. =)

-Bryan

Edited May 12, 2015 by bmyers831

[Warzouz]

You can use a SSTO rocket

The 4 orange fuel tank are dead weight in that example, but adding some RCS would only add few tons. 140T of fuel to LKO. first stage is reusable. Trip only costs less than 700 funds per ton of payload.

[GoSlash27]

The most cost-effective way to do it is by SSTO spaceplane. A good Mk.3 design can exceed 20% payload fraction and is fully recoverable at KSC. The only cost incurred is the cost of fuel, oxidizer, and RCS fuel expended during the mission.

Unfortunately, it's also the most expensive in terms of up-front cost and (arguably) the most demanding of your flying skills. Screwing up and crashing a spaceplane can be very expensive.

Best,

-Slashy

Edited May 12, 2015 by GoSlash27

[SpeedDaemon]

I have a reusable launcher made to put ~1/2 orange tank's worth of weight in LKO (18T?). I wanted a no-nonsense, fast, easy to fly lifter to build my station. It's basically a 2.5m docking port, 2.5m SAS, 2.5m service bay (probe core, batteries and solar panels), orange tank, Mainsail. At the bottom is a ring of eight turbo-ramjets and intakes mounted to small liquid-fuel tanks, and four fins. At the top are eight airbrakes, and eight parachutes.

I use a small kick from the Mainsail off the pad to get moving enough that the TRJs can breathe properly, then once I have a little altitude, another kick to go trans-sonic, at which point the TRJs go crazy. Then go balls to wall until the thrust starts giving out (you can ignore heating - won't be around long enough to matter). Kick on the Mainsail, and then turn off the jets once they're in danger of flaming out. I can get to 250km easy, no fussing about heat, balance, whatever. You probably want to be a little more aggressive turning east than with a normal rocket.

Coming back, put PE about 20-22km, and deploy the airbrakes. As soon as the jets have enough air, use them to help slow down, too. You'll probably want a little help from the Mainsail just before you touch down (~11m/s with just the chutes). Takes a bit of trial and error to hit KSC square on, but that's the only hard part. It's also not the end of the world if you lose one.

[Slugy]

Bear in mind that if you are taking tanks of fuel into space to move the fuel around you are still going to end up with empty tanks somewhere. Unless you make a recoverable delivery vehicle - which tends to take a lot of time to fly up, dock and then return, and an accident can be expensive ...

An single orange tank can be delivered by a pile of SRBs and skipper 2nd stage - which might be the easiest cheapish method.

[Falkenherz]

You are looking for a SSTO space plane.

I had just managed to finally construct a working wonderful Mk-2 SSTO space plane in 0.9 which carried 1440 units of fuel and up to six kerbals per launch (mid level tech). Since I started a new career in 1.0, the space plane tech is still barred to me, but I also suspect that my beatiful plane design wouldn´t fly anymore in the new atmospheric model.

So, at a low tech area of your space programme (just opened up all 90 point tech nodes plus two 160-nodes), my experience is that there ain´t such a thing as a efficient re-fueling in orbit. Prices are almost similar as to just launching an entire new craft which is tailored to do efficiently whatever the job is.

Edited May 13, 2015 by Falkenherz

[rizsbal]

As someone has stated on the forums you have to optimize both for in-game cost (funds) and player cost. Any design that requires hours of repetitive gameplay to achieve something is not fun to play. My cents to the problem:

Tanking in LKO is a must. It allows simpler interplanetary designs. Bigger interplanetary vehicles may be launched into orbit with empty fuel tanks or the last ascending stage (that inserts the vahicle into stable orbit) can be reused for interplanetary burn after refuel.

Tanking interplanetary ships by Mun or Minmus is not good for interplanetary missions because Oberth effect dows not work out well there. Even launch window timing is worse on a bigger orbit.

Mun or Minmus mining would be the most efficient but takes too much player time. Because the fuel must be ascended brought back to LKO and tanker must return and land on Mun each time. I would only mine satellites of remote planets for fuel (Bop mining rig is already set up).

SSTO spaceship is also efficient but too tedious to bring meaningful amount to LKO.

For me the best option is rocketing fuel to LKO, fueling the target vehicle directly and return the tanker to Kerbin surface for refunds. I use a 7 Mammoth driven 7*3 (or 7*4 dont remember exactly) S3-14400 tanks as the tanker. The ascent is helped by some SRB-s that are jettysoned and the stabilizer wings break on Kerbin reentry but otherwise the craft is reusable. The 7 Mammoth engines and the huge Kerbodyne fuel tanks return to Kerbin surface braking with parachutes and the main engines and can be recovered for funds.

A single set of the S3-14400 can be used to fuel the target the others are emptied during the ascent and return. That is 288 or 216 net tonnes of LFO. A single launch costs about 1M funds and about 420000 is returned after recovery. But I havent checked the numbers I just write them from my memory. A single launch refuels multiple interplanetary missions for me (I aim low weight for my designs).

[RoboRay]

My refuelling craft are typically just a control/propulsion module that docks to tanks and pushes them around. When the tank is empty, discard it and launch a new one to use. Think "tugboat and barge" relationship instead of "tanker".

Either way, you're ending up with empty tanks to get rid of, so why worry about keeping the one you started with?

[Pax Kerbana]

Well it depends on how you want to spend your time. In 0.90 I had a station that was constructed in 10 launches that had 6+ orange tanks worth of fuel plus 4000 mono. I built it in small launches as funds allowed and had the lifters such that a full tank was delivered with each launch.

For your case, with an existing but empty station, I see two possibilities for "fuel RFN":

1) Small, cheap launchers consisting of a liquid core stage and lots of cheap SRBs. This wil allow you to use the station as cheap as possible and fuel as you go. The nice thing about this is if you make a point to rendezvous mission ship lifters before leaving Kerbin, you can offload extra circulation stage fuel before separation and deorbit.

2) Launch I big tanker and maneuver the station to rendezvous with it. There really likey isn't a need for this unless your sending the whole station somewhere or just want your tanks to be full.

Mining is okay but at the far end of the tech tree, besides I like sending space ships on missions, not digging in the dirt. Besides, Minmus is a boring place, get your fuel from space rocks if you want to mine!

[GoSlash27]

My refuelling craft are typically just a control/propulsion module that docks to tanks and pushes them around. When the tank is empty, discard it and launch a new one to use. Think "tugboat and barge" relationship instead of "tanker".

Either way, you're ending up with empty tanks to get rid of, so why worry about keeping the one you started with?

RoboRay,

A lot has changed since my last reply in this thread back in May. Now spaceplanes are up to 40% payload fraction and extremely economical. They're definitely the way to go.

IRT your question, if you use a spaceplane the airplane is the tank, so you don't have an empty tank to get rid of.

1) launch a big honkin' tank

2) Build a station around it Bonus: Having a station means convenient orbital assembly.

3) Refill it from time to time with the spaceplane tanker.

4) Profit!

Best,

-Slashy

[Mesklin]

Most efficient way for delivering something on Kerbin orbit is air-breathing SSTO, you can use my mid-tech tanker 'Hippo' - http://kerbalx.com/Mesklin/SSTO-Tanker-Hippo-midTech

This craft can deliver to orbit 3 orange tanks of fuel.