Compressed air instead of steam in a steam engine, would that allow powered flight?

Question

My understanding is that regular steam engines could use instead compressed air (not sure what had to be done to allow this) and if so, would not such an arrangement, which did not require fuel and water to be turned to steam, would weigh a lot less I would think.

If so, would compressed air have been a way of achieving powered flight instead of/prior to the internal combustion engine which I believe was a key factor in the Wrights' success?

I believe 19th century attempts did use steam -- I wonder (as I asked in HSM but was told the question was not history-related) if compressed air was discussed or attempted and if there is any reason to believe it could have worked.

Note: I did am not asking about steam engines' ability to use compressed air -- they can. I am asking how much more efficient such a system is than using steam. I even wonder since compressing air was well understood by the late 19th whether a rocket-like usage of compressed air could be accomplished.

Answer 1

In theory, there's no reason it couldn't have worked at all. The engine part of a steam engine wouldn't really know or care that whether the high pressure gas you ran though it was steam or pressurized air. They just used steam, because it's pretty easy to store water, and boil it to get steam as a source of the pressurized gas to run the engine.

Nonetheless, let's try to keep things in perspective. For comparison, a steam locomotive typically used about 100-200 gallons of water per mile. Water expands by about 1600:1 when it's boiled. So for a steam locomotive, you need something like 1600x150 = 240,000 gallons of compressed steam per mile.

Depending on the pressure you used, you'd need about the same amount of compressed air. Let's say our aircraft needs 500 feet to take off, and we design it to run out of compressed air just as it takes off. Further, let's say it needs 1% of the power of a locomotive. That means we still need: 240,000 * 0.01 * 500/5280 = ~225 gallons of compressed air.

But that's not really powered flight, just powered take-off roll, followed by an unpowered glide and a landing.

I'm not at all sure we can extend that ever having even a hope of, say, 1 mile of powered flight. For 1 mile (plus takeoff roll), we need a pressure tank close to 3000 gallons, which (even with modern technology) weighs close to 4000 pounds. With a 4000 pound tank, our aircraft would probably need to weigh at least 4500 pounds. And to power that, we'd need a bigger engine that used more compressed air--and we're almost immediately caught in a vicious circle of bigger tank -> bigger aircraft -> bigger engine -> still bigger tank.

You can undoubtedly come up with a better estimate that this though. In particular, I've pretty much just pull the "1%" number out of the air. You can obviously come up with a better estimate by looking at the size of aircraft and speed of flight you want, and base your estimate on producing roughly the same power as some existing aircraft in that size/speed range. Likewise, you can find the minimum takeoff for that aircraft, then add the length of powered flight you want to support, and use that to get a better estimate of how much compressed air you'll need.

In the end, my guess is that yes, it's possible as long as you set your goals low enough--carrying virtually no payload, and a powered-flight distance that's probably better measured in feet/meters than in anything like kilometers/miles.

Answer 2

Compressed air engines were developed by Whitehead for torpedoes in the late 1860s. In order to get high power outputs you have to warm the expander, he developed two approaches, one by heating the motor externally, or secondly (and this is still being used effectively) by injecting fuel into the air supply for the motor, in 1904.

The main weight related problem would be the pressure tank.

So, the tech for the motor itself was available in the right time frame, your big issue is the tank.

Answer 3

Model airplanes have been run off of compressed CO2 cartridges since the 1960s, using a basic steam engine cycle. They fly for a short while, then glide. Practical for fun, but not for full-scale, long-range use.

Answer 4

An internal combustion car/truck engine is the equivalent of a piston steam engine, driven by expanding air with the expansion powered by diesel or gasoline (petrol) fuel. Jet engines are the equivalent of steam turbines with the jet powered by expanding air created by burning jet fuel.

The problem with powering vehicles is the density or the energy source. Fossil fuels have proven very effective at this for more than a century. Recently, electric batteries have proven effective as well, although their extreme weight makes them less effective for airplane use, where weight is at a premium.

Certainly we could put pencil to paper and determine the amount of compressed air we would need to power an airplane, but the plain fact is that the energy density of compressed air is pitiful compared to fossil fuels. You cannot carry enough of it to fly around for more than a few minutes.