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To bad people down voted you without commenting on why. Here's what I think: >>I mean, the Merlin-1D has really good thrust to weight ratio, but the actual am

To bad people down voted you without commenting on why. Here's what I think: >>I mean, the Merlin-1D has really good thrust to weight ratio, but the actual amount of thrust and the ISP at sea level is lacking compared to, say, the space shuttle's main engines (even with +30% the 1D is going to come short of 1,000 kN thurst, whereas the RS-25 on the shuttle was 1,860 kN). The Saturn's F1 engine had an even lower Isp, and was also KeroLox. Your focus on Isp is a bit misguided. Sure the SSME's have a stellar Isp, but guess what? The shuttle solid rocket boosters had a terrible Isp (Shuttle basically ran the RS-25 at lift off only to verify they'd start. They weren't throttled up until well into flight). When it comes to hauling millions of pounds off the launch pad, past the tower, Isp is not what you want. You want mass expelled at a tremendous rate. In other words, why not use electric ion engines with >2000 Isp for the first stage? Thrust to weight matters on lift off is why.

kllrnohj 12y ago
Sure, but the Merlin 1D is still on the very low end of the spectrum for total thrust, which you seem to ignore. You'll note I only focused on Isp for engines with similar or better thrust than the 1D. And you can't just blindly ignore this. The lower the Isp of the engine the more fuel you need and the worse the thrust-to-weight of the vehicle total ends up being. Having a very light but very low Isp engine will not get you off the ground efficiently. And if your claim to fame is being cost-effective, efficiency is kinda important. Or put another way, the RS-25 at 7,480lb each represents a tiny, tiny fraction of the shuttle's total launch weight of 4,400,000 lbs. Using a 5,000lb engine instead would change nothing. Similarly your claim they only ran at lift off to verify they'd start is wrong. The RS-25 represented 30% of the launch thrust. Even on the fairly light and small Falcon 9, the total vehicle weight is 1,115,200 lbs. The engine weight here is important but far, far from being the single most important attribute. And with a higher Isp engine with enough thrust you could then carry less fuel. Pretty sure you could spend 10,000 lbs on the engines to save 240,000 lbs of fuel (Falcon 9 has ~822,000 lbs of propellant, the RS-25's Isp is +30% the 1D at sea level)

kllrnohj 12y ago

Sure, but the Merlin 1D is still on the very low end of the spectrum for total thrust, which you seem to ignore. You'll note I only focused on Isp for engines with similar or better thrust than the 1D. And you can't just blindly ignore this. The lower the Isp of the engine the more fuel you need and the worse the thrust-to-weight of the vehicle total ends up being. Having a very light but very low Isp engine will not get you off the ground efficiently. And if your claim to fame is being cost-effective, efficiency is kinda important. Or put another way, the RS-25 at 7,480lb each represents a tiny, tiny fraction of the shuttle's total launch weight of 4,400,000 lbs. Using a 5,000lb engine instead would change nothing. Similarly your claim they only ran at lift off to verify they'd start is wrong. The RS-25 represented 30% of the launch thrust. Even on the fairly light and small Falcon 9, the total vehicle weight is 1,115,200 lbs. The engine weight here is important but far, far from being the single most important attribute. And with a higher Isp engine with enough thrust you could then carry less fuel. Pretty sure you could spend 10,000 lbs on the engines to save 240,000 lbs of fuel (Falcon 9 has ~822,000 lbs of propellant, the RS-25's Isp is +30% the 1D at sea level)