SLS vs. Falcon Heavy Raptor: Hypothetical Comparison

[dror]

Actually, the combination of lighter propellent, full flow cycle, and MUCH higher chamber pressure more than make up for the bulk density loss. Going to Raptor is about a 10% improvement. However, the biggest difference is a larger upper stage, by far. That gets nearly 25% more payload to orbit. Crossfeed does not do so well in my models.

For a fully expended all Raptor FH with larger upper stage, I get 82 tonnes to LEO with CF and 79 without CF. The same all-Raptor FH with the current sized upper stage gets 60t without CF and 70t with CF.

The best bang for the buck seems to be keeping the Merlins on the core and boosters, but using a larger Raptor upper stage. That gets 71t to LEO without crossfeed. Adding CF only gets about 1 more tonne, which hardly seems worthwhile.

All my numbers assume the boosters same the same size, volume, and weight as Block 5 Falcon 9. Orbit is 100 nm (185km) circular LEO. Everything is expended in these scenarios, but reuse makes crossfeed even more worthless and also drives strongly toward a larger upper stage.

Wouldn't the Raptor engines be heavier per unit thrust because of the more complicated turbines and higher pressure?
Could it effect your conclusion?

[envy887]

It would have less performance, since methane has less power density

It will have less energy, not necessarily less performance.

In the same engine type (P_c, expansion ratio, cycle), methalox will have lower performance than kerolox. But Raptor is far more efficient than Merlin and easily beats it despite lower bulk density and worse mass fraction. FFSC > GG, 300 bar > 100 bar.

[envy887]

Actually, the combination of lighter propellent, full flow cycle, and MUCH higher chamber pressure more than make up for the bulk density loss. Going to Raptor is about a 10% improvement. However, the biggest difference is a larger upper stage, by far. That gets nearly 25% more payload to orbit. Crossfeed does not do so well in my models.

For a fully expended all Raptor FH with larger upper stage, I get 82 tonnes to LEO with CF and 79 without CF. The same all-Raptor FH with the current sized upper stage gets 60t without CF and 70t with CF.

The best bang for the buck seems to be keeping the Merlins on the core and boosters, but using a larger Raptor upper stage. That gets 71t to LEO without crossfeed. Adding CF only gets about 1 more tonne, which hardly seems worthwhile.

All my numbers assume the boosters same the same size, volume, and weight as Block 5 Falcon 9. Orbit is 100 nm (185km) circular LEO. Everything is expended in these scenarios, but reuse makes crossfeed even more worthless and also drives strongly toward a larger upper stage.

Wouldn't the Raptor engines be heavier per unit thrust because of the more complicated turbines and higher pressure?
Could it effect your conclusion?

That's not entirely clear. Both thrust and mass scale with chamber pressure, and the more efficient cycle and longer nozzle may make up for the mass hit.

For example, the J-2X has a much lower vacuum specific thrust than the RS-25 (520 N/kg vs 670 N/kg), despite half the chamber pressure and GG cycle instead of FRSC.