Physics experiment idea: compressed air "rocket motor".

I'm calling this a rocket motor for lack of a better term (though I'm sure one of you will come up with one...)

I have the opportunity to design my own physics experiment at a University. I have this idea to do it on comparing different sizes of rocket nozzles and how it affects the thrust. Since this has to be able to be preformed indoors, for the "rocket motor" I'm hoping on just using compressed air as the "fuel". My idea is to build a cylindrical airtight chamber that is attached to some sort of pump or air compressor. And have various conical nozzles with different opening sizes that can be interchanged. I would attach this chamber to a force sensor and use that to measure the thrust produced when air is pushed through the nozzle (see diagram).


I'm wondering if this would work? I built a prototype (pictured below) and connecting it to a hand pump and pumping as hard as I could, I could only produce enough thrust to inch a lightweight DVD case across the floor, which I'm worried may not be enough to produce any usable data or see any difference between nozzles.


I'd also be interested in seeing any resources about rocket nozzles and how they affect the thrust.

Sorry if this is coming off kind of newbie-ish. I've been building and flying rockets for years now, but this is the first time I've tried to understand what some of the math behind it is.






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Add an actual propellant, water in this case. Pushing out air as good up to a point but then it runs out of 'push'. To get things moving you still need mass to eject for the Equal and opposite reaction called for in Newtonian Physics. Air just doesn't have the mass needed unless huge volumes are used .

I think you will either need to add some mass like water, or lots more air. could you use a CO2 cartridge? you could try just more pressure, a bicycle shock pump will hit 250-300psi. there are some pellet gun pumps that go really high. make sure you do your hoop stress/pressure vessel calculations!

The problem is that air doesn't weigh very much.

The density of air is 1.225 kg/m3 = 1.225 grams/liter at 15 C. If you use a 2 liter plastic soda bottle you can safely pump it up to ~10 atmospheres pressure. The bottle would contain 24.5 grams = 0.0245 kg of propellant mass. An averaged exhaust velocity might be in the region of 500 m/s, so the momentum in the flow is mv = 12.25 kg-m/s which is approximately equivalent to the impulse of a 22% D motor. If the gas discharge time is 0.050 seconds, the "motor rating" would be ~D250.

Now if you load up this rocket with water, you will get a lot more thrust. Check the web calculator to see how much more.

Use a rocket weight of 100 g, and load it with 0.001 kg of water to simulate the air rocket, and then try 0.5 0.67 and 1 kg of water and see how much higher the rocket goes.

https://www.sciencebits.com/RocketCalculator?fromForm=yes

Bob

As Bob mentioned, air doesn't weigh much.

Remember the old formula, F=M*A, that is, force = mass x acceleration. If you have little mass (such as air), you'll get little reaction force. However, if you use compressed air AND water (air pushing the water), you'll get some decent force to measure! ;-) Plus you can still do it indoors (just need to contain it).

I can recall using a 1 liter soda bottle(on wheels) for a 'woosh rocket'. fun watching it zip across the floor with a nice blue glow, did have to replace the bottle after a few runs.
Rex

Don't underestimate the amount of thrust plain ol' air can produce. If you build a proper nozzle, you will find out very quickly what I mean. The key thing is to size your nozzle to choke the flow in the chamber. Any nozzle with a throat larger than that required for choked flow is going to be pretty wimpy and will only depend on the exit plane area of the nozzle (bigger=less thrust). But size the nozzle throat for choked flow and all of a sudden your "motor" starts producing all kinds of thrust. (Side note: for a converging-diverging nozzle with choked flow at the throat, the air coming out will likely be supersonic, so...yeah...clear the way.) There's some good info on nozzles with lots of equations on NASA's website.

My one concern about using a compressor, if directly attached to the motor (rather than through a reservoir), is that you won't be able to build up any meaningful pressure in the chamber and the pressure buildup will be unsteady (if the compressor runs a two-stroke cycle), which will probably be no good for you. Also, water will just get messy. Personally, I don't think water is the best utilization of your time, but everybody above is correct in saying that increasing the mass flow increases your thrust.

I seriously do not think a soda pop bottle will hold 10 atmospheres (or bar, as they are commonly known). That's about 150 psi. I could be wrong though. I would think a soda pop bottle could safely hold up to 60 psi.

My experince with water rockets is that most soda typr bottles (not the newer lightweight water bottles) will take about 100-120psi and it varies bottle to bottle. Dont be standing nearby when one bursts as its extremely loud

rharshberger said:

My experince with water rockets is that most soda typr bottles (not the newer lightweight water bottles) will take about 100-120psi and it varies bottle to bottle. Dont be standing nearby when one bursts as its extremely loud

Click to expand...

Based on the way the line is hooked up to the chamber in his photo, I think the line would detach from the chamber before any bottle blows. I would suggest some variety of threaded fittings, but it would still require that fitting be properly secured inside the chamber. (Don't think thin-walled plastic will cut it.)

For all its worth:

I designed a compressed air bladder to power my first flying models. Used to sell them on eBay back in 2002-2003. Cheap and simple to make.

Kind of interested in the nozzle you made.

The ones used for the models first used a plastic bead as a Fill/exhaust nozzle. Later versions used a R/C fiberglass tube to give over 11-oz of thrust to a model that weighed 10oz ready to fly. The SR71 model that I made available in the cardstock thread is a continuing design from the compressed bladder days.

More than happy share a design PDF file made many years ago that I've shared on other on-line forums if you're interested.

What I did learn is that the air as it escapes would need something to push on for the reaction to work. From testing found out that simply putting a reduced size "restricter" in the nozzle dropped the thrust available. Assumed this was due to drag and reduced flow, did get not the higher pressure I was hoping for.

For every action there is an opposite and equal reaction. In compressing air there is heat, when pressure is released it gets cold. Using compressed air, all you're doing is using a form of hydraulics.

BTW-The rocket in my avatar is designed to fly on compressed air. Made a mistake in my design, it won't work. To make it work might make it unsafe to fly. Won't go there. I have several compressed air rockets that are from simple cardstock that fly very well.

A short video of launcher and rocket: [video=youtube;zajSFILs4P4]https://www.youtube.com/watch?v=zajSFILs4P4[/video]


Mike

A link to someone's version of an air powered rocket:
https://www.flickr.com/photos/jurvetson/3972922344/

Lots of experimenters been messing around with compressed air for a few years. Would love to see a nozzle design!!! Good luck with your research!

Mike