The Real Reason Doc Brown Didn't Just Make Gasoline In Back To The Future 3
As the old saying goes, if you spend two movies laying the groundwork for space-time continuum best practices, time-travel logic, and paradox warnings, you can use the third movie to have a little fun in the Wild West. Okay, maybe it's not a common saying, but it's one director Robert Zemeckis seemed to adhere to when creating Back to the Future III, the final film in the phenomenal trilogy of time-traveling adventures.
The first Back to the Future saw Marty McFly, (played by Michael J. Fox) join forces with pal Doc Brown (played by Christopher Lloyd) and travel back to the 1950s, to set up the most important meet cute of his life — that of his own parents. Back to the Future II meant a fast-forward in time, and taught audiences that you should never run into yourself in the future, for fear of putting a rip the fabric of the space-time continuum and ultimately destroying the entire universe. Fair enough.
But it was Back to the Future III that took Marty and Doc to the Old West — 1885 to be exact — for some good, old-fashioned, "how do we get out of this mess" hijinks. And when Marty rips a fuel line on the time-traveling DeLorean and runs out of gas, after getting the time machine caught in a cavalry firefight (as one does), he and the good doctor have to figure out a way to power a vehicle in the late 19th century, long before gasoline was readily available. So, the question on everyone's mind became: Why didn't scientist Doc Brown just...whip some up?
Doc Brown is a time traveler, not a chemist
Surely a scientist of Dr. Emmett L. Brown's caliber is only a few raw materials and some minor calculations away from being able to make his own gasoline, even in the Old West? Not so, claims Screen Rant, and the reason is far less glamorous than you might have hoped.
Doc Brown may be a brilliant scientist, but his genius lies in the field of physics, not chemistry. "Viewers can reasonably assume that the DeLorean has pretty specific requirements for smooth time travel," Screen Rant points out, "and hastily cobbled together gasoline made by a man who isn't primarily an experienced chemist is unlikely to pass the test."
Audiences still found this plot point sticky, as a 1993 Q&A with Zemeckis and writer-producer Bob Gale proved (via SciFi Stack Exchange). Namely, why didn't Doc and Marty just get gasoline out of the mineshaft DeLorean that Doc had hidden for his future self? The BTTF creators gave a two-pronged response: a car mechanic's reasoning (Doc would have drained the tank before hiding the car), and a time-traveler's reasoning (doing so would have risked creating a time paradox). Fans of the movie will remember that even Doc's attempted gasoline substitute, a high-octane alcohol that he acquired from the local bar, was so strong that it blew out the fuel manifold. In other words: pick whatever reason you want, we were always going to get a locomotive time machine in the end.