Rocketry is an exacting science, involving a wide variety of disciplines, encompassing everything from fluid mechanics to thermodynamics and materials engineering. As complex as it sounds, that doesn’t mean it’s beyond the purview of the average maker. [Sciencish] demonstrates this with a series of experiments on rocket nozzles in the home lab. (Video, embedded below.)
The video starts with an amusing analogy about nozzle design based on people fleeing a bad pizza. From there, [Sciencish] 3D prints a wide variety of nozzle designs for testing. The traditional bell nozzle is there, of course, along with the familiar toroidal and linear aerospikes and an expansion deflection design. Of course, 3D printing makes it easy to try out fun, oddball geometries, so there’s also a cowbell nozzle , along with the fancy looking square and triangular aerospikes too. Testing involves running the nozzles on a test stand instrumented with a load cell. A soda bottle is filled with rubbing alcohol vapour, and the mixture is ignited, with each nozzle graded on its thrust output. The rockets are later flown outside, reaching heights over 40 feet.
[Sciencish] notes that the results are a rough guide only, as the fuel/air mixture was poorly controlled. Despite this, it’s a great look at nozzle design and all the science involved. It also wouldn’t be too hard to introduce a little more rigour and get more accurate data, either. However, if solid fuels are more your jam, consider brewing up some rocket candy instead.
Good video about aerospikes. https://www.youtube.com/watch?v=D4SaofKCYwo
Doesn’t look to be consistent with parameters like fuel load, fuel/oxidizer ratio, aerodynamics. Results therefore seem suspect
the results are enough to draw the conclusion that it’s worth pursuing further with more rigorous testing procedures such as air purge followed by a pressure-timed injection of gaseous fuel such as butane (readily available, easily-managed vapor pressure but burns strongly)
A bit too strongly perhaps.
The whoosh rocket is “safe” because it has a tendency to burn fuel rich. If you start dialing the mixture towards lean, at some point the bottle will go bang on you. Methanol or ethanol allows you to tone down the burn rate by adding a splash of water to it.
Using soft drink bottles even if it does ‘go bang’ it should be pretty safe – its not a shrapnel generator the way brittle plastics would be – expect it to just rapidly turn inside out throwing large but lightweight ‘petals’ at you if your unlucky…
As always safety is relative and nothing is foolproof, but its a pretty safe experiment even if you deliberately test one of these bottles to destruction. Unless you are a real moron on your own personal safety almost trying to harm yourself, you should be safe enough.
That said I would definately start with safer fuels…
Well, let’s start with the 2 Liters of air, which is about 2.5 grams. Add 170 milligrams of butane and you have a mixture with about 8,000 Joules of energy. That’s enough to throw bits and pieces of the test stand through other stuff when it goes off wrong.
In comparison, a 9mm pistol round has about 500 Joules of energy at impact. A fraction of the energy contained in the fuel can turn some piece of the test setup into a deadly projectile – for example the screws that the guy uses for the ignition spark gap.
Thing that makes something like a pistol really dangerous is almost all the energy is confined to one single direction, pushing the pretty high mass bullet. A bottle like this failing could be dangerous, but its comparatively safe, because its not a focused blast, its putting almost no energy into any one part – in the same what you can just set off a heap of black powder and get nothing but smoke.
Also the momentum of the projectile will be pretty small every time, even the screw should it be thrown clear won’t have the same hitting power as a bullet, its both lighter, and couldn’t actually be given as much energy in the first place… Very hard to really say how much, but as there is no ‘barrel’ – nothing to help focus the energy the way of the screw its fair enough to say that all the energy is evenly distributed over the bottles surface area (yes its not but is a good enough approximation of bottle failure), so even if the screw managed to take all of the tiny fraction of energy that works on its area and not loose any at all to breaking out of the bottle or the losses you can expect as most of the energy rushes past it not being confined to a barrel its still a pathetic amount of energy, something well under 100J… Even if we take the case of a very much more minor failure when all the gasses escape via the screw hole and the bottle as a whole doesn’t absorb any of the energy the screw won’t be able to actually catch more than a fraction, its the most dangerous failure mode, but still pretty low energy in the projectile – the damn thing will be too far out of the bottle to really gain any energy damn nearly instantly, and the lack of barrel means even the portion of the blast it was actually there to catch won’t be that effective on it mostly dissipating into the free air around it..
Can such a thing harm you absolutely, but its really not that dangerous unless you act like a moron. The bottle rocket functioning properly is much more dangerous to you, its both focusing the energy into one very hot stream, and much more massive, so higher momentum than any fragment it could throw. Other than as you rightly point out the screws, and the bottle ‘cap’ nothing a failure could turn projectile really weighs enough to hit with much force… Just don’t sit on the damn thing when you launch it, or launch it off a pool of petrol… Basic safety considerations and actually taking lasting harm would be a massive freak event.