Falcon 9 Kerosene to Falcon 3 Methane

[envy887]

Raptor will probably first fly on Grasshopper/DC-X style tests of the ITS upper stage. There's no real reason to fly it before that.

Falcon 9 needs to stop changing and just fly as often as possible.

[garidan]

Not worth opening a new thread I chose this one.
Now that we are near the demo flight of Falcon Heavy and the last block 5 evolution of Falcon, do we have to wait for ITS to see fly some new hardware (Dragon 2 apart)?
Only Space X knows how much time it will take, but ITS is so big it requires a completely new infrastructure it will take years to build and get the permissions let alone fly something.
And being big, costly and reusable, it will build 2 ITS at most in the first years, to learn and improve it.
So, if Raptor is 1-2 years away from being ready to fly, why not prepare the transition to raptor and methane while building all the infrastructure?
And when finally ITS will be flying, will every payload be carried by it ? Will SpaceX still manufacture and "recondition" both Merlins and Raptors ?
I think there are time and human resources now to fill that gap and develop Raptor and knowledge on methane as rocket fuel.
Keep the Falcon dimensions so to use the same tools and processes, keep as much as possible the same height so to reuse pad infrastructure, even if with methane lines added, target only the heavy version, where the reduction of engines Raptor can bring is worth the effort and fuel volume is more fitted to methane being less dense.
With all the said constraints, you can fit 2 Raptors for each core, 6 Raptors total which are roughly 22 Merlins.
Using crossfeed you can have the central core still full of fuel when the side boosters separate to fly back (at a much lower velocity).
Probably you need less fuel (more volume than RP1 but only 6 raptors), so the central core can be lower and fit a larger second stage with raptor (it was said to require 5m diameter) and fitted for large payloads and fairings. The heavy configuration could help sustain that larger second stage using side boosters as a more robust whole frame.
Anyway initially the upper stage can be the same as Falcon, Merlin RP1, just to test while flying reliably the S2 and delivering true payloads.
Crossfeed here would be very useful to maximaze volume utilization by methane: side booster are actually an extension of the volume of the central core and preserve totally the fuel in it, which is at any effect a second stage while S2 becomes the third stage.
Crossfeed here would be easier to do, just one connection for methane line from each side booster to its own raptor in the central core. LOX for the central core engine instead would be taken from the start from the central core.
So just one pipe, one valve for each booster (just to simplify the concept).

What is difficult from the very idea of this? Reusability, because 2 engines bring to an asymmetrical one engine reentry burn. Only SpaceX can simulate this, but using 3 legs instead of four to displace the weight toward the "return engine" CG can be brought back toward the center.
If this is not enough, "baloons" inside the opposite wall in the core filled at the right moment with methane gas could displace the fuel toward the same engine, bringing some tons of fuel weight on its side and "centering" the CG on the burning engine.

This xxx heavy rocket could be lighter than Falcon heavy, having 9 instead of 12 legs, not having COPVs, having a simpler engine mount (2x3 instead of 9x3).
Performance could be good.

[macpacheco]

F9 / FH design was grounded by the requirements of expendable missions + road transportability (optimize for construction cost and ease of ground transportation).
SpaceX now has a way to fully reuse boosters and upper stage reuse is mostly a matter of having lots of spare performance (for deorbit+landing burns plus a lot of TPS for re-entry).
When we add to this the fact that despite of its higher ISP and higher T:W, raptor rockets need to be thicker so road transport need to be given up.
Even with deep cryo methane, a given volume of methane contains less impulse than the same volume of RP1. RP1 is heavier for sure, but FH/F9 design is limited by stage thickness and stretching stages further is a bad idea.
This suggests any partial usage of Raptor on F9/FH is a kludge that produce limited benefits.
The more I think about this, the more I think the best way is go Raptor all the way, to a mini ITS (meaning a big upper stage with lots of engines) or a Raptor Falcon 9 on steroids (single engine upper stage + 7-9 raptors on the booster). Either way we should end up with a boosters that might be reusable 100x without refurb and upper stages that could be reflown dozens of times without refurb too.
A new rocket that's unconstrained by road transport can achieve FH expendable performance with full reuse. In fact it should be designed without any contingency for any expendable usage. Its reuse all the way. It doesn't matter if it costs ~5x as much as a FH.
Trying to squeeze limited raptor usage in between probably doesn't save enough expended upper stages for the redesign cost. Better aim for a rocket that could be a workhorse for several decades instead.
Remember that it costed SpaceX one billion (so far) to figure out reuse. F9 Block I design costed US$ 400 million. Above that, it ties down lots of engineers that could be better used for more permanent ideas. Better to "waste" a few hundred million in 50 or even 100 more 2nd stages being wasted than divert engineers from getting a mini ITS flying sooner.
From a 1/3 thrust Raptor to 75+% thrust its probably a matter of 6-12 months more in testing/development. Better go all the way.
Besides the new workhorse rocket doesn't need to wait until Raptor testing is complete to be designed. ITS design apparently will go into high gear soon.

Remember, FH sounded easy, but it reality it was crazy hard to design. What seems simple for you or me might turn out to be very hard do actually get done.