Question: Seems like there’s a lot of new ways to power cars these days. Electricity, hydrogen, fuel cells, natural gas, turbines, and bio-fuels. What I’m wondering is why hasn’t anyone looked at rockets for powering a car. I’m asking this for an extra credit project in 8th grade science, just so you know. — BT, Dallas, TX
Answer: First, best success in your science class. You’re asking a very good question. I think we should start with what a rocket actually is and how it operates. Spend some time on that link before proceeding.
So a rocket, used as a propulsive force, burns a solid or liquid fuel that doesn’t need oxygen from the atmosphere. That’s an advantage, to a point. But solid fuel rockets cannot be throttled back. Once lit, they burn until all fuel is exhausted.
The top photo shows the setup for an experiment to see if I could use rocket power on a model of a VW bus. The long strip of wood was needed to keep the vehicle on track to pass through the photogate (black plastic structure at the end of the track) allowing me to measure its speed. The copper tube on the roof of the toy held the rocket. It was a commercially available Estes model A10-PT black powder engine (seen next to the VW in that photo). Watch what happens:
I mistakenly assumed that, for the safety of the motorist behind me, the rocket would need to be mounted on the roof of the VW. Big mistake, as you can see. Here’s the problem with that configuration:
The center of mass (COM) of the VW is indicated by the green dot. The force from the rocket is applied well above the COM. This creates a torque that rotates the VW in the clockwise direction. You can see the VW nose down shortly after the rocket ignites.
To prevent a torque from rotating the VW the rocket thrust must be collinear with the COM. So I made the following modification (right) to the original rocket assembly (left):
With the torque reduced to zero, this is how the VW performed:
That works nicely, as you can see. Unfortunately, it means the car behind you would have to stay back about 20–30 car-lengths. This would not be an efficient use of existing roads.
The biggest practical issue is that “rockets” have a finite amount of fuel and burn for a fixed amount of time once lit. They might be useful in situations where you need rapid acceleration for a brief period, similar to how nitrous oxide injectors are used on muscle cars. They could be useful in a situation where you had to pass another vehicle quickly, but they could not serve as a practical means for continuous propulsion.
Finally, since solid fuel rockets can’t be throttled or turned off, imagine what might happen if during a passing attempt you changed your mind because of oncoming traffic or a tight curve. Read this apocryphal story at the Darwin Awards.
Liquid fueled rockets can be throttled to an extent, but run most efficiently at a specifically engineered thrust. So rocket-powered cars have several problems, and the more continuous power sources you mentioned all have better promise. I say save the rockets for fireworks.
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