What would the challenges be in developing a Falcon Heavy with three or four strap-on boosters?

Question

Falcon Heavy has a standard Falcon 9 Full Thrust (FT) rocket core with two strap-on boosters derived from the Falcon 9 FT first stage. What about a heavier Falcon Heavy with four strap-on boosters (or maybe three), what kind of challenges will face SpaceX? I am wondering for some possible challenges.

With 45 Merlin 1D engines ignited at liftoff it will have a higher probability that one engine failure could cause destruction of entire rocket. But would it have problems in vibrations such as Pogo oscillation even in this case where engines are grouped in five boosters, 9 per each and not being in a single core booster connected with each other all of them (such as N1 rocket with even less engines 30 but still not successful)? The N1 had more powerful engines (NK-15) and this maybe caused higher vibrations. Probably a reason that Falcon Heavy hasn't fly yet and has many delays, could be vibrations caused in launch simulation tests. The other thing, what about the center core? Would it face to much pressure from the side boosters and by the weight of a larger second stage with more engines, more fuel and a new larger interstage? Something else mabye the aerodynamic pressure, could it be an issue? Other challenges could be in other things.

What kind of challenges would face SpaceX for such a rocket?

Answer 1

Most engine failures result in shutdown of the engine without harm to its neighbors, and Merlin appears to be a reliable and safe engine. A 45-engine cluster would barely notice the loss of one or two.

I'm not sure of the vibration implications but it seems like pogo mitigation is better understood now than it was in 1970. I'm not sure if other vibration resonance issues would be heightened or not - more mass to dampen things and a less concentrated noise spectrum versus more total vibrational energy.

Center core loading would indeed be higher at launch; I don't know if the structural strength margin is sufficient or if they would have to revise it.

Aerodynamic pressure shouldn't be an issue; assuming a similar trajectory, the force per cross sectional area will be unchanged. Aerodynamic stability shouldn't change much.

Without crossfeed, they'd want to shut down some of the booster engines as they go, but that's easily done. They could even shut some down through max-Q and then relight them, if that's more efficient than throttling down. "Asparagus" crossfeed would be best, of course, emptying the top-bottom boosters into the left-right ones while the left-right pair empty into the center, dropping one pair at a time. You'd want to flow from top and bottom to both neighbors at once, otherwise the moving fuel would exert a substantial torque, rolling the ship.

What might be the biggest problem is that Falcon is horizontally integrated - assembled while lying on its side. It might be possible to assemble the center, payload, and L-R boosters, elevate it at the pad, then attach the T-B boosters, but I think vertical assembly would be more likely.

Interestingly, ULA has released white papers showing 4-booster and even 6-booster Delta IV "superheavy" configurations, with something like 100 ton to LEO payload capacity. On one hand, this suggests that ULA thinks it's just "a small matter of engineering" to do it; on the other, ULA's business and PR practices suggest that they haven't done more than a cursory feasibility study.

In any case, even if it is doable, I would not expect SpaceX to pursue a Falcon Superheavy. F9/FH will suffice for almost all commercial payloads, and they'll look to BFR for the really big missions.

Answer 2

Pogo oscillation could be dealt with by negatively vibrating opposing engines. Like sound cancellation this would result in a total, or a net vibration of zero. The trick is to synchronize all the engines to cancel the effect of Pogo. I am not a rocket scientist, but a lateral thinker. When we ( lateral thinkers ) hit a brick wall we go either left or right. That is always better then trying to break through the wall. The system could be used on all Falcon 9 block fives and the modified falcon heavy cores. The negative effects of Pogo are cumulative, so if each of the three sections of the Heavy have no net vibration, then the Pogo effect for the entire vehicle would be within tolerances. Aerodynamically, I see no real issues with the Heavy. You treat it as an ascending lifting body design. Using the widest part of the vehicle to create additional lift on ascent. The benefit would be small, but might off-set other issues, including max-Q.