The Concentrated Fuel is a chemical.
Professor Mollicule had found a cheap way to fly up to space and grab falling satellites before they burn up due to re-entry friction.
Pr. Mollicule’s fuel is concentrated, and therefore has a lesser volume than other fuels. But because it is so tightly packed, it burns slowly. And while planes powered by it go only around 100 km/h, it allows cheap space flight.
At slow speeds, there is no risk of breaking into a space/time warp, something that always worried Pr. Mollicule.
Behind the scenes
It was featured heavily in The Duck Who Fell to Earth.