Rather than highjack another thread, I will start this one.
My basic question: has fuel ever been transferred from one vehicle to another in space?
My initial answer is no, NASA has proposed research projects to learn how to do this, but it has never been done.
As I remember it, NASA has determined that no fuel transfer will ever take place near the space station as it is considered too dangerous. OTOH, liquids and perhaps fuel are transferred to the space station during Progress dockings. I know there are delta-v maneuvers routinely done on the space station to keep it in orbit. However, I believe those are always done using one of the docked Russian vehicles. I don’t think there are any thrusters on the space station-but I don’t know.
So, what is the Straight Dope? Perhaps Stranger will be able to explain?
The Zvezda module has small thrusters that can be used to reorient the station or slowly move it out of the way of debris hazards but the orbit lifting manuevers were done previously the Shuttle and currently by Progress spacecraft. The propellants in Zvezda are hypergolic (self-igniting upon mixing) and toxic, and are therefore sealed in a self-contained delivery system before ever being shipped to the ISS.
NASA has not conducted any liquid transfer refueling operations on-orbit, nor has any other space program as far as I am aware, and while it is certainly feasible to do so it is not a trivial problem, particularly when dealing with cryogenic fluids such as liquid oxygen and liquid hydrogen, and even with more common substances such as liquid hydrocarbons which will form a rapdily form a sticky resin if volatile components are allowed to evaporate in connectors or pumps. This capability is needed, however, not only for crewed exploration (although propellant transfer is one of the requisite technologies for crewed space exploration beyond the Moon) but also to service satellites and other orbiting platforms instead of having to replace them or send up dedicated servicing craft to sustain them.
Thanks for the post, Stranger. I wasn’t aware of the specifics of Zvezda’s fuel system. I’d have sworn I read long ago about them transferring propellant on Salyut/Mir missions (ie from tanks on the cargo vehicle to one on the station).
They also have used cargo ships (and the Space Shuttle, when it was still flying) to fire up their thrusters when docked to provide a boost in orbit to the station. If they’ve got some left over fuel, might as well put it to good use rather than take it back to Earth.
The demonstration did perform liquid transfer (ethanol, in this case), though being a demo it was only to a mock satellite. The focus of the mission seems to be as much with remote valve manipulation as much as anything.
It’s not clear how much their experiments speak to large-scale cryogenic propellant transfer. There’s an enormous set of problems here, not least of which is that any cryogenic propellant is in a constant state of boil-off, and in microgravity you are likely to end up with a kind of Swiss cheese blob which doesn’t easily feed into the transfer port.
Rockets have this problem when restarting, and fire “ullage” motors–small thrusters which fire to move the propellant rearward to the feed tube. Once the main engine is firing, the ullage motors are no longer needed since the acceleration keeps the propellant in place. But for an orbital transfer, you don’t want to fire the main engines, or even small ullage engines. So you might use tidal or centrifugal forces instead, but that has its own set of tricky dynamics. All of this is solvable but it’s going to take work.
Flexible bladders are used in small propellant tanks for storable fuel–hydrazine, etc. Works just fine. But for cryogenic fuels in a constant state of boil off, you still need to do something about the gases (either venting or recondensing them).
Also, scaling a bladder up to BFR sizes (9 meter diameter, 15-20 m long) sounds like a challenge. Full-size propellant tanks have a lot of stuff in them that could get in the way–structural ribbing, anti-slosh and anti-vortex baffles, various piping, etc. And in the base of the BFR ship specifically, it has additional “header” tanks for landing.
There are other options, such as tanks that use surface tension to direct the propellant to the feed. But a lot more research is needed.
Flexible textile or metallic diaphrams work fine in small spherical tanks with propellants under moderate pressure and temperature (although it is not possible to get all propellants out) and the amount of ullage gas required to pressureize the backside of the diaphram is not a major inert mass penalty. For a large propellant tank suitable for fueling a crewed interplanetary spacecraft (hundreds of tons of propellants) a diaphram just isn’t very practical. Nor is a piston very practical; pressure tanks have spherical (or quasi-spherical) ends for structural reasons. A flat ended piston, therefore, will leave a hemispherical amount of propellant in the tank available, and a sphereical ended one will leave a full sphere of tank volume unusable (and also contribute to some very adverse dynamics). There have been suggestions of having the tank itself act as a bellows or contracting flexible membrane but there don’t exist any materials which have both the requisite flexibility and durability to make a tank of useable size.
There is also the problem of the coupling; as observed in the 2015 pad explosion that destroyed a Falcon 9, propellant loading can be hazardous even in relatively predictable conditions on Earth, and doing so in the widely fluctuating temperature in space, as well as a lack of gravity in freefall to settle adverse dynamics, plus the work that is done to force propellants through a small aperature which results in heating and the potential for phase change or other adverse interactions with materials makes transferring propellants in orbit problematic and requiring a lot of engineering development and testing.
Some proposals for future on-orbit fueling systems use gelled or solid state fuels which can be transferred controllably even in freefall conditions and close ot vacuum with minimal losses, and then converted into a fluid by heating or catalyzation. There are a few gelled propellants that are being developed for upper stage vehicles where the slosh dynamics of fluids are undesirable but they all have performance compromises over even relatively low performing hydrocarbon liquid fuels and hydrogen peroxide or nitrogen tetraoxide oxidizers, and so aren’t well suited for interplanetary propulsion.
Absent of a rich source of propellant materials in space, liquid chemical propulsion for crewed interplanetary propulsion is a very marginal proposition at best. The low specific impulse performance and low payload mass ratio make them a pretty poor option that exposes crew to cosmic space radiation and a higher chance of solar particle events for much too long to be suitable for transporting supposed ‘colonists’, and wholly unsuited to operations to collect space materials from near Earth objects or sources in the asteroid belts or Trojan nodes. A smarter effort for supporting true crewed space habitation and exploration would focus on developing solar electric, nuclear electric or fission salt propulson systems which could use water, dissociated hydrogen, or even atomized silicates as a propellant instead of trying to sythesize liquid chemical fuels to a necessary degree of purity for use in rocket engines.
ASTRO transferred 14 kg of hydrazine monopropellant into NextSat as a demonstration for on-orbit refuelling of satellite stationkeeping systems. This is in no way comparable to the hundreds of tons of cryogenic oxidizer and liquid fuels that would have to be transferred to a spacecraft for a crewed interplanetary mission.
Not for something like BFR, and it wouldn’t allow you to “top off” already partially-filled tanks, but it would be an interesting design to explore for satellites and such. Imagine if after a few years a satellite could get a visit from space’s equivalent of a robotic AAA and just swap out the depleted fuel tank for a full one, and get back to satelliting for another 5 or 10 years.