Fuel crossfeed

[Temstar]

So I was reading Wikipedia pages on boosters to pick up design ideas and came upon this:

Falcon Heavy will be the first rocket in history to feature propellant cross-feed from the side boosters to the center core. Propellant cross-feeding leaves the center core still carrying the majority of its propellant after the side boosters separate. This gives Falcon Heavy performance comparable to that of a three-stage rocket, even though only the single Merlin engine on the upper stage requires ignition after lift-off, further improving both reliability and payload performance. Should cross-feed not be required for lower mass missions, it can be easily turned off.

Engines from all three cores light up at launch. But until fuel runs out in the booster cores, the main core uses little or none of its own propellant. There are two separation events: booster separation and main core separation. This is akin to a three stage rocket and thus enables greater performance. Compared to what is thought of as a two and a half stage rocket, like the Delta IV Heavy, the central core can operate at full thrust and still be left with a full fuel load after booster separation, as opposed to a partial load.

The first though that came to my mind was 'Hey! They stole my idea!'

Okay seriously, how come it\'s taken the aerospace industry so long to use this idea in real life? I started realizing fuel crossfeed was the best thing since sliced bread only a week after playing KSP. Surely this is mature technology by now? Space shuttle\'s been using fuel crossfeed from the External Tank to the Orbiter for 20 years now.

[Resender]

Cause the idea in real life is not as easy as you might think

[Ascensiam]

You can bet it\'s been on the drawing board for ages, the case is just that unlike KSP, you\'re faced with a \'\'do it right or lose billions\'\' problem, so they had to be sure of all alternatives, and make sure this was the best one in the end.

[sal_vager]

Also in KSP we don\'t have to worry about the amount of flow this system would need, or any leakage should something go wrong.

I\'m glad to see space x try this though, it seems to be the most efficient system possible in our beloved game and if it can work in real life it will seriously cut down on the size of rockets and the amount of fuel they need.

[Temstar]

After doing some more research turns out this is quite an old idea:

In 1947, Mikhail Tikhonravov developed a theory of parallel stages, which he called 'packet rockets'. In his scheme, three parallel stages were fired from lift-off, but all three engines were fueled from the outer two stages, until they are empty and could be ejected. This is more efficient than sequential staging, because the second stage engine is never just dead weight. In 1951, Dmitry Okhotsimsky carried out a pioneering engineering study of general sequential and parallel staging, with and without the pumping of fuel between stages.

I wonder if we\'ll see http://kerbalspaceprogram.com/forum/index.php?topic=10468.0 for real in the future.

[Zephram Kerman]

It is my favorite configuration! Having all engines working, instead of hauling dead weight, really gives a nice improvement of efficiency.

Furthermore, you can cascade these cross-fed 'packet' rockets to use every stage of your craft. (I think this was the reason you linked to the Melissa monster.) Here\'s a simpler version of the concept:

[colmo]

I\'ve been inspired by the Asparagus approach to fuel crossfeed to increase efficiency, but the moment I try scaling it up, the early stages destabilise the rocket when the outer tanks start running empty, even though the ship is radially symmetrical.

Instead of 1 central section, and 6 outer, I used a tricoupler triple central section, and 9 outer (3x3). In each set of 3 outers, the first stage fed the second stage, the second fed the third, and the third fed the tricoupler. The rocket works from the 3rd stage onwards, but the first two stages do not - it\'s definitely the empty tanks that destabilise it, as it launches fine and flies ok for a while.

[Temstar]

Instead of 1 central section, and 6 outer, I used a tricoupler triple central section, and 9 outer (3x3). In each set of 3 outers, the first stage fed the second stage, the second fed the third, and the third fed the tricoupler.

If the instability is specifically a tendency for the rocket to roll I know what causes it. It\'s caused by the bottom of your stage 1 and 2 osculating from side to side. If these osculation are synchronized it induces a roll force on the rocket. To fix you have to attach struts right at the very bottom of the outer 9 rockets to the center 3.

[colmo]

If the instability is specifically a tendency for the rocket to roll I know what causes it. It\'s caused by the bottom of your stage 1 and 2 osculating from side to side. If these osculation are synchronized it induces a roll force on the rocket. To fix you have to attach struts right at the very bottom of the outer 9 rockets to the center 3.

I believe I had everything pretty well strutted. The rocket did roll, when the first stage started running low on fuel.

I\'ll try it again with the struts even lower.

Is it significant if you run fuel lines from or to the bottom, middle or top of a three-tank stack?