Dr. Robert H. Goddard with his complete rocket with the double-acting engine in November 1925, following more than two years of pump development based on the idea of a separate pump for each propellant. Dr. Goddard made an important change in his pump technique by combining both pumps into a single double acting unit. Though gas pressure, rather than pumps, was used on his first successful liquid-propellant rocket shot of March 16, 1926, the idea of combining both pumps into a single unit led to a successful solution of the pump problem and hence marked a significant advance.