Nope. You cant use *JUST* diodes because the on-board battery would supply the power as the off-board battery is at a lower voltage.
Also its desirable to minimise the weight of the circuit permanently attached to the rocket. If your system can tolerate 34V on its incoming DC bus, the optimum solution would be to use a 3A, 50V rated Schottky diode in series with the supply from the on-board battery + use a boost converter with the go-cart battery to get a regulated 34V, which would be applied after the Schottky diode, and would keep it reverse biased so there is no load on the on-board battery while powering your system. The boost converter's rectifier diode will prevent significant reverse conduction through it so you probably don't need an extra diode in series with its output. Without a relay, you wont get a momentary loss of power as it switches over from external to internal power, and the only extra weight on the rocket is the Schottky diode in series with the on-board battery + the external power connector (+ associated wiring), as the boost converter stays on the ground with the 24V battery.