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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

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Questions › 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
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Garmaine asked 4 years ago

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?