For his thesis, he carried out balloon flights to the top of the atmosphere of a cloud chamber triggered by a scintillation counter to study the charge distribution of primary cosmic rays.

At this height, the rockets would ignite and shoot up to 350,000 feet, carrying equipment intended to study cosmic rays and particles trapped in Earth's magnetic field.

The same year, McDonald combined the scintillation counter of his thesis with a cherenkov detector into a balloon instrument that not only provided a novel measurement of the energy spectrum of primary cosmic ray helium nuclei, but also served as a prototype for devices carried on many spacecraft.

In that role, he helped start a NASA partnership with historically black colleges to fund research projects by faculty members and graduate students.

After McDonald retired from NASA in 1989, he joined the Institute for Physical Science and Technology at the University of Maryland, where he was a senior research scientist until his death.