Does thrust to weight matters more than ISP?
RD-180 has been considered as pinnacle of kerolox rocket engines, using the most efficient staged combustion cycle. It has been often remarked as far superior engine compared to the modern kerolox engine - Merlin 1D. Its ISP at sea level is 311 dwarfs 282 reported by Merlin 1D. It uses special metallurgy able to sustain its oxygen rich cycle. There is only one little area where it is not so good as Merlin: thrust to weight. T/W ratio is 78.44 compared to 183 at sea level.
So the question is whether better T/W ratio can compensate for lower ISP? And what is the relationship between the two? Since the relationship between dry/wet mass ratios and stages is a complex one, it can be easier to compare the difference in total impulse provided by a rocket engine.
Higher T/W essentially means that rocket engine can carry more propellant per unit of mass when launching. So twice the thrust, twice the propellant mass. which means, twice the factor in the full to empty mass ratio. So ISP is not everything. T/W has correlation with mass ratio, another key factor in rocket equation. This correlation is not linear, since more propellant mass also require more structural mass (for example larger tanks).
If we use Falcon 9 as an example vehicle, replacing nine Merlin 1D engines with appropriately scaled RD-180 would increase engine mass (dry weight) for 5.600kg on the first stage and 600kg on the second stage. In total approximately 1.2% of the whole stack. But it would increase total payload around 5.000kg (assuming similar ISP increase on second stage engine). So we can conclude that ISP indeed does matter more than T/W.
So the question is whether better T/W ratio can compensate for lower ISP? And what is the relationship between the two? Since the relationship between dry/wet mass ratios and stages is a complex one, it can be easier to compare the difference in total impulse provided by a rocket engine.
Higher T/W essentially means that rocket engine can carry more propellant per unit of mass when launching. So twice the thrust, twice the propellant mass. which means, twice the factor in the full to empty mass ratio. So ISP is not everything. T/W has correlation with mass ratio, another key factor in rocket equation. This correlation is not linear, since more propellant mass also require more structural mass (for example larger tanks).
If we use Falcon 9 as an example vehicle, replacing nine Merlin 1D engines with appropriately scaled RD-180 would increase engine mass (dry weight) for 5.600kg on the first stage and 600kg on the second stage. In total approximately 1.2% of the whole stack. But it would increase total payload around 5.000kg (assuming similar ISP increase on second stage engine). So we can conclude that ISP indeed does matter more than T/W.
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