I think we’ve all been pretty dazzled by the new Gordon Murray Automotive T.50 hypercar — and I normally find that word kind of eye-rollish, but in this case I think it’s justified. It’s such a pure exercise in technology employed to make driving really exciting: small, featherweight, an advanced, naturally-aspirated V12, carbon body, and, maybe most excitingly, that big-arse fan on the back. Now, the fan is for complicated aerodynamic purposes, but there’s something I can’t help but wonder: could you drive the car with only the fan?
I know it’s kind of an idiotic question, but when you have a big fan exhaust on the back that’s blowing out air and looks like a jet engine, it’s really hard not to imagine it pushing that car along. And it’s not like the fan doesn’t generate any forward thrust — we know it generates about 15 kg of thrust, because Gordon Murray told us so.
So, if it’s generating thrust, I think it should be able to move the car. The question is really how fast could it move it? I know 15 kg of thrust isn’t much, but if I recall, thrust is cumulative — that’s why those NASA probes with the little ion engines that make as much thrust as a piece of paper exerts on a desk can eventually get those probes moving so fast.
I realised I needed to do some actual maths here, and I also know my limitations, of which I have so so many, including not being great at maths. So, with that in mind, I reached out to Jalopnik’s semi-tamed physicist, Dr. Stephen Granade, to see if we could figure out — at least roughly — if the fan can actually move the car, and, if so, how fast could it move it?
Here’s what Stephen had to say:
I love that you looked at a car that’s the pinnacle of Gordon Murray’s career, that should hit 97 km/h in under three seconds, and said, “OK, sure, but could you push it with its fan?”
That’s not a ridiculous question. A fan is just a propeller in a hoodie. And like an aeroplane’s propeller, the T.50’s fan produces thrust. When it spins, it accelerates air backwards, which pushes the fan, and the whole car, forward.
It’s complicated to figure out how much thrust a fan produces. The further out on a fan blade you go, the faster it moves through the air. It’s like how you get dizzier on the edge of a merry-go-round than in the middle. That changes how each part of the blade interacts with the air. While I can look at pictures of the T.50’s fan with its seven straight blades, without knowing the actual blade shape, I can’t calculate its thrust.
Lucky for us, GMA told us it can produce 15 kg of thrust. Is that enough to make the car move? Probably so. If I put my ancient Honda Fit’s gear stick in neutral, I can make it roll on level ground by sticking my foot out of the door and pushing, and my car isn’t anywhere near a hypercar. It’s more of a hypocar.
But saying “probably so” and calling it a day isn’t any fun. You know what is fun? Maths.
Thrust is a force, which equals mass times acceleration. Divide the thrust by the T.50’s mass and we’ll know how fast the fan can accelerate the car. The T.50’s extremely light, with a curb weight of under 2200 pounds (1000 kg), less than my Fit. If you’re fine with the fan pushing an unoccupied car, then it can accelerate the car at a blistering half a foot per second per second. That’s 0 to 60 in around 3 minutes.
But even small amounts of thrust add up over time. In space, NASA’s used ion thrusters that barely produce enough thrust to lift four pennies, but over a long period of time changed the spacecraft’s speed by nearly 40,234 km/h.
Great! Let’s get a fan-pushed T.50 up to ludicrous speeds! Unfortunately, by working in space, NASA avoids two things: friction and air resistance.
Friction may keep us from getting the car rolling in the first place. The fan’s thrust has to overcome the wheel bearing’s at-rest friction, and the tires’ deformation adds to that friction.
If we have to, we can get around that by giving the car a hard push ourselves, though mind the fan! The fan should then keep the car rolling forward. As it picks up speed, though, air resistance climbs. Roughly speaking, when you double a car’s speed, the aerodynamic drag force goes up by four. Even for a low-drag car like the T.50, at some point the air will push back as hard as the fan pushes forward.
A recent numerical analysis of the drag force on a very crude “sports car” CAD model indicates that we’ll reach that point around 77 km per hour. But without knowing the T.50’s aerodynamic characteristics, I can’t tell you how right that number is.
Of course, there’s one way to find out for sure. Is there $US3M in the Jalopnik budget for a T.50?
OK, so, let’s recap:
Yes, the Gordon Murray T.50 could propel itself with only the thrust from its rear-facing fan, producing 15 kg of thrust. With a curb weight under 998 kg and factoring in some degree of rolling resistance, we’re looking at acceleration of 15 centimetres per second, every second, or a 0 to 60 time of about three minutes.
Except it likely couldn’t quite get to 97 km/h.
As far as top fan-only speed goes, if the aero of the car is roughly in the ballpark of that racing car model used in that Polish research paper (it’s probably not that far off) then we can expect a top speed of around 77 km/h before the drag on the car limits the acceleration. Let’s be generous and say that the T.50 probably has better aero, so we can maybe push that to 80 km/h.
So, there you go! One of the fastest cars you can buy (well, probably not you, or me, or anyone I actually know) becomes one of the slowest if you don’t let it actually drive its own wheels directly. Then again, if you add those no-drive-wheels criteria to almost any other car at all (with the possible exception of a the Helicron), then the T.50 once again proves to be faster than almost anything else.
Now, I want to be clear that we do not yet have official numbers for the T.50’s coefficient of drag or frontal area; I’ve reached out to GMA and have yet to hear back, but I’m hoping now that this story is up perhaps they’ll understand the gravity of this and if they can get me the proper numbers, we’ll update the chart and data accordingly.