Comparing Long March 5's CZ-5-300 RP-1/LOX booster to Falcon 9 first stage; similar Isp's but very different relative volumes

Wikipedia's CZ-5-300 and Falcon 9 Full Thrust provide the following rough figures.

                           CZ-5-300     F9 First stage
length      (m)              27.6           42.6
diam.       (m)               3.35           3.66
gross mass (kt)             156.           433.
thrust (SL, kN)            2400.          7607.
burn time   (s)             180.           162.
Impulse/volume (kt/s/m^2)  1776           2750


Both use RP-1/LOX which is not as unusual for a side booster as I originally thought.

If I very crudely estimate mass-specific impulse and volume specific impulse for these using sea level thrust times burn time I get

                         CZ-5-300     F9 First stage       comments
Impulse/mass   (m/s)       2770.         2846.           similar
Impulse/volume (kt/s/m^2)  1776.         2750.           big difference


Their sea level Isp's are similar, but the CZ-5-300 has only 2/3 the impulse per unit volume. Is there anything meaningful to this difference (extra stuff inside) or is this likely trivially explained or otherwise unimportant?

• Did you calculate volume from booster length and diameter or is it a fuel volume? Unused volume within the booster may be very different when common versus separated bulkheads are used.
– Uwe
Dec 29, 2019 at 18:59

Is there anything meaningful to this difference [?]

It shows that it's a much smaller stage with a hat.

1. The height of the Chinese booster includes its top cone. A quick measurement on the pictures gives its height as 4.5m, leaving a useful height of 23 m.

2. The stage is smaller, so the contribution of the engines to its height is larger. The length of the nozzle won't change much and might even be larger compared to the 9 rather small bells of the Falcon. The Merlin engine has a length of 3 m, so let's assume it's the same for the booster. That leaves us with a remaining height of 20 m vs. 39.6 m.

Now, recalculate the impulse to (useful) volume ratio: 2450 to 2950, much closer than before - just 17% difference.

1. The Chinese booster likely uses regular RP-1 and O2, not sub-cooled like the Falcon. The upgrade from Falcon 9 1.1 and FT gave them about 15% more dV performance (just a back-of-the-envelope estimate based on thrust and burn time).

Taking this into account, we're at a 2450 to 2500 ratio. Concluding, there is not much room for things like

extra stuff inside