[
ISP (vacuum) – 321 sec. Not even close to an RL-10 on a Centaur. May be with a nozzle extension they could get to 350s? May be. An that's with an extra 30% volume over RP-1.
That's still maybe a 25 to 54s increase in ISP over the Castor-30 but the real savings would be in the dry mass fractions of the stage.
A methane stage of equivalent size would have a lower dry weight then a Centaur due to the smaller tanks.
An improvement over the Castor, probably. The Castor 30A has a very respectable 91.30% fmp. To put that in perspective:
US: fmp/isp
Solid:
Castor 30A: 91.30%/292s
Vega Z9: 78.91%/294s
Hypergolic
Cyclone-4 US: 77.93%/330s
Ariane 5 EPS+ 80%/321s
Proton-M Briz-M: 88.79%/326s
Proton-K Block DM: 86.05%/352s
RP-1/LOX
Soyuz-2A Block I: 91.52%/325s
Soyuz-2B Block I: 91.52%/359s
Zenit-2 2 stg: 89.68%/350s
H2/LOX
Ariane 5 ESC-A: 76.65%/446s
Saturn IV S-IVB: 89.31%/421s
Delta IV US 5m: 87.72%/462s
Centaur 4m x 1:
92.76%/
450sThe Centaur is the best US in fmp and second on isp. Only to the DVIUS, which is almost the same engine with a nozzle extension. Talking about the RL and the Centaur is getting to the very best. A potential replacement for the Castor 30, probably better. Surely better at higher energy orbits. But you would have to include a nozzle extension, the Chase as is is 61 T/W. So you'd have even worse performance. And I really think making an 87% fmp CH4/LOX US without a balloon tank design is very good. Input your numbers assuming 87%/340s vs 91.30%/292, assume a normalized weight of the stage of 14,000kg and a payload's weight of 5000kg. You'd get 219m/s extra delta-v at 5,000kg. That's a 5700kg payload to LEO. But the Castor XL will do 6000kg. So you'd need something quite more powerful. And I haven't considered the gravity losses, since the Castor 30A has 3.5 times more T/W for this stack.
