How to Build and Launch Compressed Air Powered Rockets
Compressed air rockets can be a lot of fun. Here's how it's done:
Compressed Air Rocket Project Overview
After having successfully built a hair spray powered potato launcher, I watched a program on the Discovery Channel about the punkin( chunkin( competition. I noticed that most participants in this competition used compressed air to power their launchers. In thinking over the use of compressed air, it seemed to also be a reasonable choice for a project on a much smaller scale: the compressed air powered rocket.
Compressed Air Rocket Launch Base Design
Next, the launch pad was designed and built. The design really didn't consist of much; this was more a 'just do it' quick project with safety as a main consideration. The base was built from scraps of MDF (medium density fiberboard) which was coated with a mixture of polyurethane, Minwax walnut varnish, linseed oil, and naptha. I have used this coating mixture in the past, and it penetrates wood deeply (especially with extra naptha added), and it protects against water.
Next, a length of 1/2" trade size PVC (polyvinyl chloride) schedule 40 pipe was assembled with a few elbows. A hole was drilled in the launch base to fit the PVC pipe through. The PVC pipe was then fitted into the launch base and secured with a couple screws and plumber's tape.
Launch Pipe Selection and Construction
Schedule 80 PVC pipe or conduit could have been used, but the inner diameter of the pipe is quite small. We wanted to keep the pressure drop through the air line as low as possible to provide maximum force to the air rockets.
Then, a 1/2" NPT (national pipe, tapered thread) fitting was attached to the end of the PVC pipe. The PVC pipe was cemented.
A 1/2" NPT to 5/8" hose barb fitting was then screwed into the PVC pipe assembly. A length of 5/8" garden hose was used for a length of the air line. Yes, garden hose is not intended for compressed air use - another safety consideration.
Air Line Selection
The length of garden hose was then attached to a 1/2" Parker air quick disconnect fitting to interface to the shop's compressed air system.
The garden hose was then attached to the shop's compressed air system. A length of the shop's 1/2" Goodyear compressed air hose with quick disconnects was attached to the compressed air system.
The compressed air in the shop runs around 120psig.
Prepare the Air Rockets!
Next, several air rockets were constructed from 1 and 2 litre soda bottles. Typical 1 and 2 litre soda bottles with a narrow mouth fit snugly and perfectly onto the 1/2" PVC pipe. It is a perfect fit for the compressed air rockets.
Then an air rocket was fitted onto the launch pipe. The compressed air valve (a 3/4" valve) was then opened. POP! was the sound made as the compressed air rocket launched into space. Ok, well 30 feet or so into space anyway. It was actually quite interesting to watch these little soda bottles fly up in the air.
We also found that filling the 1 litre bottles about 1/4 full with water before placing them on the launch tube yielded the greatest achievable height.
How Much Force was Behind These Air Rockets?
That is an excellent question, and one that is perfect for OzScience.com! Alright, let's take a look. The soda bottle openings were roughly 3/4" diameter, (or 3/8" (.375") radius).
- A=(pi)(r)(r)
- A=(3.14159)(.375")(.375")
- A=.442 square inches
Our applied compressed air pressure is estimated at 100psig. The shop supply was about 120-130psig, so considering pressure loss through the hose and piping, we figured about 100psig was available at the rocket.
So, with .442 square inches of area, and 100 pounds per square inch applied, the approximate rocket launch force was (100psig)(.442 sq.in.) = 44.2 pounds of launch force. That's not too shabby.
Granted, the air rockets got that force for only a very short duration. Let's say 0.1 seconds by estimate.
What is the Acceleration of the Air Rockets?
To calculate the acceleration of the air rocket, we need to know the weight of the rocket. I figure the empty one litre bottle weighed roughly one once.
So now we know that F=mA (force = mass * acceleration). So solving for A, we get F/m.
I like to work in metric units for force and acceleration to avoid confusion. So 44.2 pounds force comes out to 196.6 Newtons. The bottle weight of one ounce works out to about 0.0283 kg.
- F=m*A
- A=F/m
- A=(196.6N)/(0.0283kg)
- A=6947 meters/second/second
- So at 9.8m/s/s for gravity, that is nearly 709g!
Awesome! So What Was the Air Rocket Launch Speed?
Now let's estimate the air rocket launch speed when it leaves the launch tube.
- v = vi + at
- v = 0 + (6947N)(.1s)
- v=694.7 m/s
So that's a launch speed of 694.7 meters per second, which is about 2,279 feet per second.
Now 2,279 feet per second seems a bit high. After all, that is supersonic. Now we did get quite a pop when the rocket launched, but it wasn't a sonic boom type sound. 2,279 feet per second high power bullet speed. But we neglected the friction of the bottle against the launch tube. That no doubt took quite a bit of the launch force away. Also the rockets were not very aerodynamic, so they decelerated very quickly. But they did launch pretty fast.
Also, the estimate of 100psig at the launch tube is probably high. The reason is the opening time of the valve and capacitive effect of the air hose. It is not possible to hand open a valve fast enough for this type of application; this fact was well demonstrated in the Mythbusters chicken gun experiments. The launch tends to occur before the valve is even fully open.
So if we figured the launch pressure was more like 33psig, that would take us to 1/3 of 2279 ft/s or 759 ft/s. That is still supersonic in cold weather anyway. But considering the friction of the bottle against the tube probably cuts that force in half again, so figure an actual launch around 350 ft/s.
Compressed Air Rocket Safety
It should go without saying that safety is absolutely a first thought in a project such as this. In any project there is the possibility for injury or property damage, and fotunately we were able to avoid both in the execution of this project.
Safety was considered in the compressed air rocket project design. People were not allowed within 20 feet of the launch rig when it was in operation. Safety goggles were worn by the operator. The operator kept his hand on the compressed air control valve at all times, so as to be able to quickly shut off the compressed air supply if something went wrong.
The biggest safety risks in the project were flying debris due to the compressed air, possibility of major hose whip if the hose ruptured, and the possiblity of explosion of the air rockets themselves.
Yes, we did use PVC pipe with compressed air in this project. Typically PVC pipe should NEVER be used for compressed air applications, especially where it is exposed to any type of oil (such as from an oil lubricated air compressor), or where it is exposed to sunlight. PVC can and will rupture in compressed air use, and it often sends out lots of shrapnel. Compressed air holds a lot of power and its sudden release can be quite explosive.
This article is simply a documentary on my compressed air rocket project. It is for entertainment value only; such a project can be dangerous and should not be attempted.
Post new comment