Thank you for your reply, tfar. I'll try those squeezing method. There was no refillable cartridge that came with mine (it's the smallest model) but I will squeeze a bit of the ink out of a cartridge prior to flight next time.

My Waterman rollerball leaked in flight, so I thought, what the heck, let's carry a fountain pen since even a rollerball dares to leak!

It's just not the same writing with a ball point, although it's a nice ball point...