SpaceX News

[paperburn1]

Once again the Falcon design has a approach that takes this into account and make the gain from gantry feeding not worth the effort.
The Falcon heavy has a cross feed system that allows the center tanks to retain a large portion of the fuel when the saddle rockets run empty.
This allows the rocket to shed weight sooner allowing more weight to orbit

[kunkmiester]

Which is great for later in the launch. Most rockets burn for a few seconds to spool up the engines, check for malfunctions, and actually get the rocket moving. How much fuel does the 9 burn before actually lifting off?

It probably isn't worth it, because of the complication, but if you have the valves and hookups for using the crossfeed, 90% of the trouble is done. It's an additional to go wrong among other things.

[paperburn1]

About 3 tons of ox/fuel or 1% if my math is right

[JoeP]

A mechanical boost pushing off the pad might work be better or at least safer than feeding fuel and oxy in for those critical moments.. Like the Navy had to do with launching planes off a carrier's short runway. Maybe even some steam pistons the give the rocket a bit of a boost for, say, maybe 25-75 feet on a some kind of simple vertical rail-guide. Probably way too expensive to implement though -- considering the extreme mass, force, loading, and timing involved.

[KitemanSA]

Adding extra "pony" boosters per your post above.

Sorry, your statement doesn't make much sense. Why would auxilliary, neutrally boosted, flyback tanks add weight during recovery?

[KitemanSA]

I'm thinking more of a Falcon 9 Enhanced. The returnable F9s have three landing legs, why not have three tuned F1s that are strapped on to, or between, the legs but transfer no thrust. They go along for the ride and are equiped to fly back. All they do is transfer fuel. Unless of course it is easier to actually transfer the thrust.

Having strap on fuel tanks that are ejected befor the first stage exaustion is a way to increase fuel to structure weight, but it is dependent on adequate thrust from the active first stage. Falcon 9 first stage has about 1.2+ million pounds of thrust. And the stack weight is (guess) about 1 million pounds. If you had two strap on fuel tanks, each with perhaps 500,000 lbs of fuel, the stack weight would be ~ 2.0 million pounds or more. Upon mission ignition the rocket would go nowhere until 800,000 pounds of propellant was burned off. Then the rocket would slowly begin to climb with initially dismal efficiency. If you add weight, you have to add available thrust, unless you already have lots of surplus thrust capacity. This is what Falcon heavy does by having the two strap on "Fuel tanks" that have their own rocket engines. This maintains, or probably actually mildly improves the overall thrust to weight ratio. You are wasting less of your fuel for just balancing out the Earth's gravity. You may also achieve some advantage by transferring some of the fuel from the strap on powered "fuel tanks" so that the primary first stage has more fuel on board upon "tank" separation than it would otherwise.

The proposal was to have a set (2 or 4, depending on mission) of Falcon 1 enhanced, i.e., longer, self thrusted, and depending on optimization results either have it's excess thrust transfered into a strengthened F9 core or simply use it as a neutrally boosted, fly back, auxilliary fuel tank so that when ejected, the core would be full of fuel. That would leave the core with more fuel to fly back with itself.

[ladajo]

Adding extra "pony" boosters per your post above.

Sorry, your statement doesn't make much sense. Why would auxilliary, neutrally boosted, flyback tanks add weight during recovery?

I don't understand what you are saying here. What does "neutrally boosted" mean?
My point is; if you have more stuff, you have to push it up as well, that takes more gas, if it provides more net thrust, then what you gain is overall capacity. Is that what you are looking for? A bigger rocket?
If you are going to recover it, then that adds mass as well that must be accounted for on the front end for the initial boost. If you are going to passively land it with parachutes or other drag devices, then it is mass to support the decell. If you are going to actively land it, with engine decel, then that also takes mass in the form of the engines, and more importantly fuel. Which again, adds mass up front for the boost phase, and must be accounted for.

I really am not sure what you are looking for overall, more capacity in cargo? delivery mechanics options? Increased recovery envelopes/options?
No matter which you are looking for, it all adds up to more mass to boost up front with commensurate gains in fuel (not linear), and more mass either as a single entity or multiple to decell for recovery be it active or passive both of which require fuel and push capacity up front, and in the case of active, additional fuel on the back end for any mass groupings being recovered, which in turn (non linear) adds more fuel/burn up front as well. There is a point in the mass balance where there is no longer any gain and you start losing mission capacities.

[krenshala]

While I fully agree with what you are saying about increasing mass, and thus fuel requirements, it is my understanding that SpaceX is (or at least was) planning to do fuel cross-feeding from the outer "booster" cores to the central core during flight with the express purpose of having the central core (nearly) fully fueled at the point when the boosters drop off. Having all engines burning, but keeping the central core fully fueled until all the boosters drop off gives a significant boost in delta-v or a boost in payload mass-fraction.

[Skipjack]

While I fully agree with what you are saying about increasing mass, and thus fuel requirements, it is my understanding that SpaceX is (or at least was) planning to do fuel cross-feeding from the outer "booster" cores to the central core during flight with the express purpose of having the central core (nearly) fully fueled at the point when the boosters drop off. Having all engines burning, but keeping the central core fully fueled until all the boosters drop off gives a significant boost in delta-v or a boost in payload mass-fraction.

Last I heard, they are going to do that later, but wont be doing it for the initial flights.

[paperburn1]

From spacex press release.
Three cores make up the first stage of Falcon Heavy. The side cores, or boosters, are connected at the base and at the top of the center core’s liquid oxygen tank. The three cores, with a total of 27 Merlin engines, generate 20,418 kilonewtons (4.59 million pounds) of thrust at liftoff. Shortly after liftoff the center core engines are throttled down. After the side cores separate, the center core engines throttle back up to full thrust.Each of Falcon Heavy’s side cores, or boosters, is equivalent to the first stage of a Falcon 9 rocket with nine Merlin engines. At liftoff, the boosters and the center core all operate at full thrust. Shortly after liftoff, the center core engines are throttled down. After the side cores separate, the center core engines throttle back up.

PROPELLANT CROSS-FEED SYSTEM
For missions involving exceptionally heavy payloads—greater than 45,000 kilograms or 100,000 pounds—Falcon Heavy offers a unique cross-feed propellant system. Propellant feeds from the side boosters to the center core so that the center core retains a significant amount of fuel after the boosters separate.

THREE NINE-ENGINE CORES
Inside each of Falcon Heavy’s three cores is a cluster of nine Merlin engines. These same engines power Falcon 9, enabling efficiencies that make Falcon Heavy the most cost-effective heavy-lift launch vehicle in the world. With a total of 27 first-stage engines, Falcon Heavy has engine-out capability that no other launch vehicle can match—under most payload scenarios, it can sustain more than one unplanned engine shutdown at any point in flight and still successfully complete its mission.


http://www.spacex.com/falcon-heavy

[KitemanSA]

Adding extra "pony" boosters per your post above.

Sorry, your statement doesn't make much sense. Why would auxilliary, neutrally boosted, flyback tanks add weight during recovery?

I don't understand what you are saying here. What does "neutrally boosted" mean?
My point is; if you have more stuff, you have to push it up as well, that takes more gas, if it provides more net thrust, then what you gain is overall capacity. Is that what you are looking for? A bigger rocket?
If you are going to recover it, then that adds mass as well that must be accounted for on the front end for the initial boost. If you are going to passively land it with parachutes or other drag devices, then it is mass to support the decell. If you are going to actively land it, with engine decel, then that also takes mass in the form of the engines, and more importantly fuel. Which again, adds mass up front for the boost phase, and must be accounted for.

I really am not sure what you are looking for overall, more capacity in cargo? delivery mechanics options? Increased recovery envelopes/options?
No matter which you are looking for, it all adds up to more mass to boost up front with commensurate gains in fuel (not linear), and more mass either as a single entity or multiple to decell for recovery be it active or passive both of which require fuel and push capacity up front, and in the case of active, additional fuel on the back end for any mass groupings being recovered, which in turn (non linear) adds more fuel/burn up front as well. There is a point in the mass balance where there is no longer any gain and you start losing mission capacities.

Neutrally boosted means that each "tank" is actually an F-1 that has its own rocket engine that keeps up with the core and neither adds nor subtracts thrust to it, unless they want it to. At initial take-off, the Aux-tanks and the core all get their fuel from the aux-tanks. When the aux-tanks reach their assigned "empty" condition, they seperate from the core and fly back. Yes, they have additional mass, for themselves, but the cost of the fuel to land them is negligible. What the aux-tanks do is allow the F-9 R to launch heavier payloads and still get back to land and get re-used.

[ladajo]

So what we are really saying here is that we would strap on pony tanks with a net positive thrust to account for extra fuel mass that will transfer to the central core. Then they would detach and fly back on their own.
What is the real difference between the STS method of solid fuel pony boosters that did the lion's share of pushing, and the SMEs were throttled down to save gas while the boosters were in play? If you wanted more lift/main fuel savings, they could have added more booster power by doubling up on each side. Of course, the external tank would probably have needed some beefing up, but certainly doable with a similar end result.

[kunkmiester]

I'd imagine at some point you get diminishing returns.

Also, isn't SpaceX planning on making a Falcon 9 size rocket with a single of their larger engine? Plus the 3 core version, plus they want to scale the Falcon 9 design to use nine of those. I suppose a 5 core Falcon nine super wouldn't be completely unreasonable, but how useful would any intermediates between Falcon Heavy and whatever they're calling the next be?

[Skipjack]

I'd imagine at some point you get diminishing returns.

Also, isn't SpaceX planning on making a Falcon 9 size rocket with a single of their larger engine? Plus the 3 core version, plus they want to scale the Falcon 9 design to use nine of those. I suppose a 5 core Falcon nine super wouldn't be completely unreasonable, but how useful would any intermediates between Falcon Heavy and whatever they're calling the next be?

Those were old plans. None of their known current plans are going into this direction anymore.
Reasons:
engine out capability, engines not able to throttle low enough for powered landing

[paperburn1]

The three core currently exceeds the lift capacity of the shuttle.
see the graphic at the bottom of the link
http://www.spacex.com/falcon-heavy