2/ X hence!::. f is approximately 2f, and we see that for large distances round trips are nearly four times as expensive as one-way trips! Knowing the answer, it is now clear why this should be so! For a round trip, it is necessary to set up very substantial fuel depots at the far end of the desert, whereas for two one-way trips, the closer one gets to the far end of the desert, the smaller the depots have to be.