Good points there...
I dunno about pressure fed designs... That's one of those paradigms that just has never caught on... kinda like "big dumb booster"... (hehe... always thought that was kind of a stupid sounding name... hard to get Congress to pony up when you ask them for funding to build something with the word "dumb" in it... LOL
The problem with pressure-fed engines is the chamber pressure is SHARPLY limited by the need for the TANK PRESSURE TO BE HIGHER. Therefore, you're stuck with either LOW PRESSURE ENGINES (and chamber pressure and ISP are intrinsically related via fuel burn efficiency and exhaust velocity/temperature of combustion and all that) or you up the pressure to gain some more power and efficiency out of your engines, but RAPIDLY run into a wall of MASSIVE TANK STRENGTHENING, meaning your tank weight soon eats up all your payload, OR BOTH (massive tanks AND underpowered low-efficiency engines). There is a reason that pressure-fed engines have always pretty much been last in line except for when absolute simplicity and "it's GOT to work FIRST TIME, EVERY TIME, NO MATTER WHAT" type reliability is required (like SPS, LM descent/ascent stages, OAMS, etc.) where efficiency or power is at best a secondary concern. That's why pressure fed hasn't been developed for first stage propulsion that I can recall...
Not saying you can't "brute force it"... If you've ever read about "Sea Dragon" it's well worth a read-- there's a thread about it the HLV section at nasaspaceflight.com/forums... HUGE multi-million pound SINGLE PRESSURE FED RP-1/LOX engine on both stages, ~ a million pounds to orbit depending what version you're looking at... built out of ordinary steel and aluminum BY SHIPBUILDERS; basic pressure-hull type construction, not finely tuned highly refined aeronautically engineered structures-- cheap and dirty, with the operative word being CHEAP! That was Bob Truax's idea, anyway, too bad it was never given a shot (though I saw something awhile back, a video, of a rocket towed out to sea, stood vertical in the water by flooding a ballast tank under/behind the engines, and then launched from in the water... kinda like a floating SLBM... So why not?? Seems like the most realistic method to launch a large 10 m lbs plus rocket... simply because of the acoustics impact... NASA won't be launching anything over about 11 million lbs thrust from KSC simply because they could never afford to replace every window for 100 miles around after every launch... it's either build a "super heavy" floating launch complex farther out at sea or, go to the 'tow it out to sea, float it upright, fill the tanks, and launch" type paradigm...
At any rate, perhaps with better materials and technology pressure fed could be made to work-- FLOX or some other exotic propellant combinations, or perhaps thrust-augmented nozzles that essentially add 'afterburners' to a rocket engine and multiply the thrust, might be just the ticket around the 'low pressure engine' efficiency problem.
As to recovery of niobium nozzles or other things, SpaceX seems to be working toward that goal... working on first stage recovery (which will always be easiest due to the velocities and altitudes involved are so much lower) and even talking about possibly recovering second stages from near-orbital velicities and altitudes... (I'll believe it when I see it...
) Until then, tossing niobium nozzles on expendable upper stages doesn't seem to worry them too much (or hacksawing off the bottom of the nozzle if it cracks! LOL
)
It'd be interesting to see a modern S-ID version by SpaceX... SpaceX is really in a great position, by developing a highly efficient fairly inexpensive kerosene liquid engine based on pintle injection... I'd LOVE to see a 1.5-2 million pound class Merlin 2 engine on a Saturn-ID stage setup... Heck for that matter I'd love to see the numbers on such a setup powered by RD-180's... The RD-180's and the Merlin 2 would UNDOUBTEDLY put the F-1 to shame... I freely admit to having been an unrepentant "F-1 hugger" for a long time... UNTIL I really started reading up and found out that the F-1 is VERY outdated, and it's performance and design has been eclipsed in nearly every area except perhaps in pure raw maximum thrust (at least for the 1.8 m lb F-1A-- RD-180 can already almost match F-1 for thrust). RD-180 is a MUCH more efficient engine ISP-wise than F-1, which would make a S-ID THAT MUCH MORE CAPABLE than the design was with F-1! Presuming Merlin 2 would have the same ballpark efficiency (or higher) than RD-180, and have more thrust (it's supposed to AT LEAST be a match for F-1A, at 1.8 m lbs thrust) that would make a S-ID a virtual SLAM DUNK! If the thing could lift off under the power of the outer four engines GROUNDLIT and the center engine AIRLIT, with a more vacuum-optimized nozzle, performance would increase substantially... but note that S-ID's performance even with ALL REGULAR GROUNDLIT SEA-LEVEL EXANSION RATIO F-1's (which robs performance at altitude) was still NOTHING TO SNEEZE AT!
Combine ET tank super-lightweight construction mass fractions in the core sustainer stage, with high-thrust kerolox booster engines in a jettison thrust structure with high thrust- high efficiency kerolox boosters and you'd have a world beater IMHO... nothing could touch it! Try the idea out with Merlin 1D's as a subscale test vehicle...
That I'd love to see...
We're going to HAVE to figure out biconic heat shields sooner or later anyway, if we ever plan to actually land folks on Mars... the masses and sizes are simply too huge for regular solid spherical section heat shields to ever work or be feasible to launch from Earth's surface-- unless you make them folding or ballutes or something esoteric like that... A biconic can be launched as the nosecone/payload fairing of the vehicle. They basically glide (but at a very low lift-drag and very low cross-range-- maybe 'controlled fall' is a more correct term... If large downmass is a 'requirement' at some point, teaming up to prove biconic research at the same time just makes good sense...
OL JR