Fuel Transfer

[supermap]

In KSP you can transfer liquid fuel and oxidizer whenever you want, but what happens when you want to do it in real life.

Would it be hard to transfer liquid hydrogen, liquid oxygen, RP1, etc?

Which would be easier or which are just not used in space for long missions?

[Kryten]

All the fuel transfer that's been done has been between space stations and supply craft, using the 'storable' fuels (i.e. liquid at room temperature and don't degrade) nitrogen tetroxide and hydrazine. RP-1 would degrade over long periods of time, and LO2 would require refrigerated tanks and much bulkier refrigerated piping.

[supermap]

thanks for the clear answer

does this mean that all spacecraft use dinitrogen tetroxide and hydrazine (or another non cryogenic) for any kind of main thrust? (not LV´s)

or do some use cryogenic fuels, (i dont mean for fuel transfer) since i guess it shouldnt be too hard to keep them as cool in space right??

[Kryten]

All deep-space or long-term ones yes, unless they use ion. It's not easy to keep stuff cool in space due to the unfiltered sunlight, experiments with storage of LO2 in space with sunshields have been proposed but not tried yet.

[sneakeypete]

Its also worth mentioning that for real world proposed designs where fuel is transferred during launch, such as the falcon heavy, the rate of fuel transfer is also a major issue. There's a lot of fuel that you need to move between tanks in a short time, which requires heavy pumps.

[Rune]

Its also worth mentioning that for real world proposed designs where fuel is transferred during launch, such as the falcon heavy, the rate of fuel transfer is also a major issue. There's a lot of fuel that you need to move between tanks in a short time, which requires heavy pumps.

Errr... those big pumps are part of rocket engines by default. They are called turbopumps, and all rockets that aren't pressure-fed have them. The issue are the fuel lines (and what happens if they leak), and the momentum that moving that much fuel that fast creates IRL. No asparagus in the real world, the configurations have to be symmetrical so the forces cancel each other out.

As to orbital fuel transfer, it's only been done with storables, but there are a couple of tests/ideas planned to make cryogenic fuel transfer and storage work. For deep space, my personal favourite is Methane/LOX: similar moderate temperature, low boiloff rate, amazing isp, and ISRU potential in pretty much anywhere, especially Mars and asteroids.

Rune. SpaceX is apparently developing a Methane/LOX upper stage engine, too.

[Scotius]

In preparation for Mars missions, perhaps? Planetary Resources and Deep Space Industries should invest in this technology heavily too, if they really mean to build any industry in space.

[Rune]

In preparation for Mars missions, perhaps? Planetary Resources and Deep Space Industries should invest in this technology heavily too, if they really mean to build any industry in space.

My favourite personal prediction: MCT (the acronym shrouded in secrecy Musk has mentioned several times) means Mars ¿Colony? Transport, and it's a single stage vehicle that goes on top of a Falcon heavy, and can land on Mars (half aerobrake/half propulsively) with enough payload to carry a ISRU plant, refuel there, and then make it back to Earth throwing nothing away. Mars and back for a few hundred million dollars a flight. Redundancy can be achieved by launching several on each window, in case you are worried about safety.

Payload margins are tight for that (~5km/s for MTI+Mars capture+Terminal braking means mass ratio around 4, which means 13.25mT payload+rocket, essentially the same weight class as Dragon), but then again, they might have in mind an even bigger rocket (~100mT) to launch it, Musk has also hinted that. The thing I'm sure of is, it could do a lunar mission by default.

Rune. How do you like my crystal ball?

[nothke]

It is not actually hard to transfer fuel in space if you have well designed mechanism..

The actual problem with fuel transfer in KSP that is unrealistic, is the breaking of Newton's third law.. Every action has a reaction. So, if you move fuel from one side of the ship to the other, you are moving the mass, therefore, the ship would react and move into the opposite way. Imagine you were on a small boat and you tried to jump to another boat, the boat you are jumping from would move back and most likely, if you have E in physics, you would splash in water in between the boats as a punishment. Now what you see in KSP would be as if the boat was nailed in stone, the ship doesn't move at all. Therefore, you can exploit this error like Scott Manley did in that fuel transfer video.

IRL, fuel transfers would probably be slow cause you don't want to make ship jump around during transfers. The same reasons why robotic arms move so slow.

[Scotius]

Rune. With lifting capabilities of Falcon Heavy a whole lot of things will become possible Super Dracos look very promising too. But best of all: SpaceX actually actively works towards their goals and have cold, hard results to show. Rest of space companies? So far most of them is all talk.

[nyrath]

Kryten agreed, cryogenic fuels are a pain. I've seen lots of proposals for orbital depots carrying cryofuels, they have lots of foil umbrellas to shade from the sun, and absolutely none of the proposals has actually been created and flown.

But astronautical people are working on them, because such depots are the key to opening up the solar system. The ability to refuel at your destination drastically cuts your spacecraft's required mass ratio, so you no longer have to make your tanks out of metal foil and your structural members out of metal soda straws.