Space Fuel Stations

[Spacenstuff]

Hi, another crazy idea from myself I'm doing a project, so keep coming up with stuff...

A lot of rocket launches aren't using maximum lifting capacity of rockets. What if rockets would install a fuel tank in second stage that would be filled with the difference between satellite weight and maximum capacity of rockets.

We would get maximum capacity out of each launch and could create fuel station in LEO/GTO and Lunar, Mars orbits. Whenever a rocket reaches destination, it would transfer all the excess fuel to these "fuel stations" and later could get it back for return trip.

Seems like a lot of saved money.

Second part of idea was to upgrade most of Falcon 9 launches to Falcon Heavy and use all the spare capacity to bring more fuel up in the orbit.

This would also create contingency for space flight - something happens, you always have a fuel station near you

[e of pi]

Spacenstuff,

The idea has been talked about before of sending some sort of "useful ballast" along on missions to utilize any unused capacity. In a sense, this is what secondary payloads like cubesats or smallsats riding with a primary payload are, though there's a utility to sending propellant that's different than the benefits of having to involve another spacecraft. (No one has to "build" propellant, there's less matchmaking, the properties are easier to specify and analyze generally.)

There's a few problems. The biggest is that right now, we don't have much of an infrastructure to use propellant placed by any kind of method--this is basically a propellant depot network filled/topped by excess launch capacity to nearby orbits. Since we don't have a practiced way to do prop depots, the kind of people who would be heavily interested tend to focus on those challenges--demonstrating zero-boiloff storage, the logistical questions of transfer and settling propellant, etc. In addition, launching ballast propellant like this has a few additional problems that dedicated tankers wouldn't. Payloads launch to a lot of orbits, and even those we think of as "similar" aren't all that close to one another.

LEO comes in many inclinations, and even those in the same inclination can be relatively far from each other in real maneuvering terms. For instance, the two "rings" of Treasure Planet from the Disney movie show something that happens with LEO comsats--two orbits with the same inclination, but which have their ascending nodes in different locations, leading to radically different actual orbits. A launch into one of the "ring" orbits would be poorly positioned to load propellant to a depot at the other. A similar thing happens with geostationary orbits depending on what longitude the apogee is at. Thus, the second stage would have to have the endurance to do additional burns to manuever radically to rendezvous, or you'd need a lot of depots each consolidating smaller amounts of propellant, or the propellant transfer depots would have to spend a lot of time going out of their way to retrieve this "free" propellant. As Heinlein said, there ain't no such thing as a free lunch.

You also have to wrestle with preventing the propellants (typically cryogenic) from boiling off while the second stage waits for rendezvous with the depots without adding excessive mass--something a single purpose tanker can more easily justify.

In a future with more orbital infrastructure and experience in propellant transfer, it might be worth doing. For the moment, it's got a lot of complexities hidden in the details that make it a lower priority than just demonstrating propellant launch and transfer at all from single-purpose tanker launches.

[Spacenstuff]

Some really good points.

What if you have like 10 stations in different orbits and second stage "fuel tanks" use electric propulsion to meet up with closest one. Then it can leave the fuel or take fuel on board and slowly adjust its orbit to a future launch vehicle orbit.

Creating an automated system where the excess fuel would find nearest "fuel station".

Problem with ride sharing, as in small sats example, people are lazy and they won't look for partners on regular basis. Few million is nothing for these massive companies, but with a standard system benefits could compound into something noticeable.

An as with any crazy new idea, there is obviously lack of expertise that is why it hasn't been done.

To be honest I think ULA was considering something similar with second stages (now that I start to think about it), but with SpaceX launch cadence,  they could make way bigger contribution. Unfortunately they will have too much on their hands with BFR, so doubt this will ever happen.

[TrevorMonty]

Instead of starting a new thread why not revive one of the many Fuel Depot threads.

[Spacenstuff]

Because:
A)Why not?
B)By the fact that I have 5 posts, it is evident that I'm new to the forum

Thanks for challenge anyway

[e of pi]

What if you have like 10 stations in different orbits and second stage "fuel tanks" use electric propulsion to meet up with closest one. Then it can leave the fuel or take fuel on board and slowly adjust its orbit to a future launch vehicle orbit.

That's sort of what you'd need to do, but it create complications. Pushing the task of rendezvous and docking on the second stage means it needs a lot of additional hardware, making it less of a stage and more of a spacecraft. It'll need much larger endurance (days or weeks instead of hours or days), it may need solar arrays for power (instead of just short-duration batteries), better communications, an electric propulsion system, docking apparatus, thrusters, and avionic "brains". All this adds cost to an expendable stage, and for a reusable stage it means that the cycle time between missions goes up because it spends longer in space after dropping the primary payload.

Additionally, you start having a lot of depots if you want to put one easily in reach from any given orbit. That's a lot of depots, and means that while you might be launching several hundred tons of propellant this way, any given depot might have only a few dozen or less. That means each individual depot is less useful, and a mission picking up propellant may need to hit multiple depots to gather the propellant for a mission beyond that point in space or that each depot needs to be larger than can easily be filled from excess mass on missions and be filled at least in part, possibly even majority from purpose-operated tanker launches.

None of these are unsolvable, but the conditions you need for it to make more sense than a few depots filled from specific tankers are somewhat constraining:

-Launches need to be frequent enough that you can accumulate significant propellant in a variety of orbits and depots via this method
-Launches need to be expensive enough that every wasted kilogram a rocket could lift but didn't is painful
-Additional stage/depot hardware needs to be cheap enough that you don't worry about throwing out an ion engine and solar arrays every single launch, or launch fleet for a reusable upper stage needs to be large enough that you're not worried about tying them up in orbit making depot runs before returning
-Systems need to exist or be planned for development which are capable of making use of depots, particularly a number of small depots (large systems filling from multiple depots before a departure, or many smaller but more frequent injections using a single depot to go wherever the users of the propellant are going, whether cislunar or beyond)

[johnfwhitesell]

A lot of rocket launches aren't using maximum lifting capacity of rockets. What if rockets would install a fuel tank in second stage that would be filled with the difference between satellite weight and maximum capacity of rockets.

It's called ACES and I wish they would do it as soon as possible rather then spending time on their reusability program.

[sunworshipper]

That's sort of what you'd need to do, but it create complications. Pushing the task of rendezvous and docking on the second stage means it needs a lot of additional hardware, making it less of a stage and more of a spacecraft. It'll need much larger endurance (days or weeks instead of hours or days), it may need solar arrays for power (instead of just short-duration batteries), better communications, an electric propulsion system, docking apparatus, thrusters, and avionic "brains". All this adds cost to an expendable stage, and for a reusable stage it means that the cycle time between missions goes up because it spends longer in space after dropping the primary payload.

That seems very inefficient.  What if instead of a second stage travelling to a fuel depot, the depot has an tanker which travels to the second stage with exactly the amount of fuel the stage requires.  And then the tanker returns to its depot or another.  Same thing when the second stage is making a deposit.

[thammond]

That's sort of what you'd need to do, but it create complications. Pushing the task of rendezvous and docking on the second stage means it needs a lot of additional hardware, making it less of a stage and more of a spacecraft. It'll need much larger endurance (days or weeks instead of hours or days), it may need solar arrays for power (instead of just short-duration batteries), better communications, an electric propulsion system, docking apparatus, thrusters, and avionic "brains". All this adds cost to an expendable stage, and for a reusable stage it means that the cycle time between missions goes up because it spends longer in space after dropping the primary payload.

That seems very inefficient.  What if instead of a second stage travelling to a fuel depot, the depot has an tanker which travels to the second stage with exactly the amount of fuel the stage requires.  And then the tanker returns to its depot or another.  Same thing when the second stage is making a deposit.

A dedicated tanker would likely be more efficient.  Guess it all depends on if a tanker would use more fuel transferring back and forth than the amount it recovers.  Also the tanker would probably need to use some form of a ullage motor to settle the fuel during transfer ops which costs more fuel.

[johnfwhitesell]

Minimizing fuel losses is probably less important then streamlining hardware.

[thammond]

Minimizing fuel losses is probably less important then streamlining hardware.

I think sunworshippers suggestion of a dedicated tanker was intended to streamline hardware by having most of the additional hardware required be on the dedicated tanker.

My comments were more in line with fuel usage not fuel losses.  A dedicated tanker would use fuel to go from the fuel depot to the rocket stage and then use fuel to go back to the fuel depot after removing the fuel from the rocket stage.  Plus probably need to use fuel in the transfer of fuel from the rocket stage to the dedicated tanker.

The fuel recovered from the rocket stage would need to be more than the fuel used by the dedicated tanker as described above for it to be a feasible plan.  Just need to do the math so to speak.  But each launch would be a unique case (ie amount of extra fuel, difference in orbital parameters between fuel depot and rocket stage, etc).

[Jim]

Because:
A)Why not?
B)By the fact that I have 5 posts, it is evident that I'm new to the forum

Thanks for challenge anyway

Because that is what you do.  You look for old thread first.

[Sam Ho]

Because:
A)Why not?
B)By the fact that I have 5 posts, it is evident that I'm new to the forum
Thanks for challenge anyway

Because that is what you do.  You look for old thread first.

Here's a previous thread about propellant depots:

Over on the "Griffin Blames Atlas for Ares Dissent" thread the discussion of propellant depots has started to take on a life of its own, so I started this thread so that topic can be pursued and the original thread get back to Mike Griffin. I am going to ask Chris to grab all the relavant posts and move them here, then I'll post a link to here from that thread in case anybody goes looking for them.