Then there’s the matter of SpaceX’s planned flyback reusability. This requires highly variable thrust to be available in the flyback parts of the booster stage’s flight regime. Nine engines allows more fine-grained control than would three or five. SpaceX is optimizing more than one thing at once with the Falcon 9’s design.

As to your completely unexplained basis for believing SpaceX to be on the ragged edge of disaster in producing so many Merlins, it’s worth pointing out that the Merlin’s parts are produced on CNC machine tools. Many users of such machines like to run them constantly at or near their limits in the belief this somehow maximizes production. What it mainly does is burn up tooling and machines. I have no knowledge of SpaceX’s machining philosophy, but I suspect they’re smart enough to have figured out one can get much more consistent quality out of machines and tooling that are not run at the limits of their performance envelopes.

I also don’t know to what extent the assembly of Merlin engines is a robot-centric as opposed to human-centric process. But the assembly process at Tesla is highly robotic and a rocket engine is both smaller and has a lot fewer parts than a car. I wouldn’t be in the least surprised to learn that Merlins roll off the line virtually untouched by human hands.

If your concerns about SpaceX’s engine production are based on familiarity with how part fabrication and assembly are done in traditional aerospace, it would be good to remember that SpaceX is not traditional aerospace.

Complex how? It has a structure connecting the center engine to the tankage, just like any single-engine booster, and a ring of eight additional engine mount points much like the center one and feeding thrust loads into the tankage in a similar way through a common triangulated thrust structure. It can’t be all that complex or the Falcon 9 would not cost so little compared to other launch vehicles.

and thrust vectoring,

SpaceX seems to use hydraulic pistons in what looks to me in pictures I’ve seen to be a Stewart Platform configuration to orient the axes of its engines. That seems to be how other boosters steer too.

low thrust to weight ratio,

As the Merlin 1-D has the highest thrust-to-weight ratio of any sizable rocket engine ever built, I assume this baseless canard is simply a matter of your obdurate refusal to believe anything SpaceX says.

high base heating,

I have no idea how you imagine you know this.

limited growth potential

Current Merlin 1-D’s produce almost twice the thrust of the original Merlins and may have additional “stretch” left in them. The Falcon 9v1.1 has more than 25% greater payload capacity than the v1.0 bird, even after allowing for reusability margins.

And pay the payload tax when they are not running.

They all run when launching a payload. After stage separation, though, only three run to stabilize the booster and fly it back toward its point of departure. Only one engine fires to soft-land it. There’s no “payload tax” associated with running variable numbers of engines on the return to launch site leg of a mission. The payload and the booster parted company already.

I never liked this approach.

You’ve never had a good word to say about anything SpaceX does. So what else is new.

It seems sub-optimal.

It’s quite optimal – for what SpaceX is trying to optimize – namely long-term cost of operation. You, no doubt, have something entirely different in mind, like. say, payload mass. And you’re probably supposing that maximizing it is equivalent to optimizing it.

The F9 engine cluster results in complex mounting structure and thrust vectoring, low thrust to weight ratio, high base heating, limited growth potential…

reusability more easily by running only as many engines as needed at any given stage of flight.

And pay the payload tax when they are not running. I never liked this approach. It seems sub-optimal.

Had a good time here at Singapore the last few days. Got a tour of the new Indian CV. very very nice. up and coming power. RGO

You can see the baleful results in the failure histories of various launch vehicles. The Ariane 5 has one core-stage engine. That vehicle had four failed missions out of its first 14 before settling down and building its current enviable track record. The Falcon 9 profited hugely by using an evolved version of the engine SpaceX started trying to fly Falcon 1’s with back in the day. When SpaceX moved on to Falcon 9, they deliberately set up Merlin 1-C/D production as a mass-production industrial process. The results speak for themselves. Merlin engine production is not the limiting factor in SpaceX’s fabrication of Falcon 9’s.

Only one Merlin engine has ever failed on a Falcon 9 mission, a 1-C model on the 1.0 version of the vehicle. On the five Falcon 9 v1.0 missions, Spacex flew 50 Merlin 1-C’s. On the four v1.1 missions to-date, they’ve flown 40 Merlin 1-D’s. Combined, that’s appreciably more engines flown than Arianespace has flown Vulcains on Ariane 5’s.

ULA’s Atlas V, incidentally, benefits from the same effect. By the time ULA started buying RD-180’s for their rocket, the Russians had already worked all the bugs out of their production process. Thus, the Atlas V was spared the usual few early-in-program failures that tend to crop up when new vehicles with new engines are introduced. The RD-180 was not a new engine by the time it first flew on Atlas V.

Finally, the reason the one Merlin 1-C failure that did occur didn’t scupper the mission was precisely because there are nine engines on the Falcon 9 1st stage. Fault-tolerance through use of multiple components that mutually back one another up is old hat in computer engineering, but aerospace types seem to prefer obsessing over minimizing total part count. Not always a good idea.

Multiple engines also make it possible to do reusability more easily by running only as many engines as needed at any given stage of flight. This way, individual engines don’t have to be able to be throttled to absurdly low power levels. It’s tough to make a single million lbf engine throttle down below 10%. If, you’ve got nine engines, you can get pretty much the same effect by just shutting eight of them off and throttling the remaining one much less radically.

Raptor, therefore, is not very likely to appear solo, or even in mere tandem, on any SpaceX vehicle except, like the vacuum version of Merlin 1-D, on an upper stage. I suspect most Raptors, like most Merlin 1-D’s, will be clustered – quite likely in groups of nine once again – on a stage 8 to 10 meters in diameter that will have total thrust roughly 20% in excess of that of Saturn V’s 1st stage. In combination with a suitable upper stage, such a vehicle could put 150 metric tons or more into LEO. A triplexed “heavy” vehicle using three such cores might put 500 tonnes at a time into LEO. Just as with Falcon 9, engine redundancy would insure against loss of mission for all but very unlikely structural failure, explosive and/or multi-engine failures (3 or more of the 9).

… the only thing/variable I would add to this is thaat along with reusability SpaceX has got to figure out how to “point and shoot” meaning launch on a regular basis with none of these “hiccups” that postpone launches.

To be fair, some of the problems have been beyond SpaceX’s control, e.g. the range fire and now the Orbcomm customer requesting a delay.

My guess is CRS-4 will be delayed from August because Orbital ORB-2 has pushed back to July due to the Antares engine test failure.

I think everyone involved understands why SpaceX isn’t launching once a month. But the purpose of SpaceX is to try to launch once a month, to bring down the cost, to make spaceflight more routine. In my opinion, we need to cut them some slack, so long as they don’t make a rash and irresponsible decision that leads to the loss of life or payload.

Time to say, Goodnight Gracie.
Goodnight George.