How can I take hi-speed photos of water rockets?

Question

How can I take a picture of water rockets as they fly through the air? What settings would be best under normal daylight conditions? What is the slowest shutter speed I can use that the lighting will still look natural, but it will also capture the water trail and droplets? And what would be the best aperture so that the background is not in focus, and the main thing in focus is the rocket?

Asked by J. Walker

Answer

"Real" water rockets accelerate EXTREMELY fast. While some with restricted orifices may stagger off the launcher, anything that aims at performance accelerates faster than almost anything else you are likely to photograph. A top rocket 9by calculation and experience) accelerates at up to about 100 g !!!.

There are various shots you can take. Looking up at the rocket well above you is a fairly general shot and panning can work. Exposures and shutter speeds can easily enough be arrived at by experiment.

Getting a good shot as the rocket leaves the launcher up to say 10 to 15 feet in the air, takes care, planning and luck. Smaller aperture for better depth of field helps. Fast shutter speed can help BUT there is another way. Set camera to manual, adjust for a relatively low shutter speed - below max flash sync speed, and then add flash. Rear curtain flash is even better if you have it. Adjust flash to ambient lighting so rocket stands out when the flash operates. Good shots may be had by framing from launcher to about 15 feet pointing upwardsish from near the ground so the rear curtain flash catches the rocket sharply. You get a water trail with some blur (depending on shutter speed) and a sharp rocket IF you are lucky. Just using flash plus ambient can also be good.

An interesting point to take photographic advantage of is that a rocket which has the majority of its mass as propellant and which is not very restricted in its jet will accelerate to faster than its own exit velocity. (The rocket equation makes this clear when you look at it and plug in high Full/empty mass ratios). Using an eg 1.5 litre bottle with about 100 to 120 psi and bottle mouth as orifice and 1/3 fill (about optimum) the water will be all exhausted by about 15 feet after takeoff BUT the rocket will exceed its exhaust velocity after about 5 to 10 feet from the ground. This means that water below this point is being ejected with a downwards velocity BUT above this point the ejected water actually has a net upwards velocity. The end result is that the water plume in the air changes shape as the water exiting slows and then reverses. Above the reversal point it starts to fan into a cloud with increasing spread as the upwards velocity of the water increases with height and is spread more by th air as it hits it faster. When I first saw my photos of this I did not understand what I was seeing but it makes sense once you realise. The effect is amazing - quite unlike what you expect to see and fast enough that you don't really notice with the eye alone unless you are looking for it. So the photos are especially surprising.

I can find some photos of this effect if of interest. Quite some years ago now :-)

Also fun is a two or more photo sequence of the ascent with the photos manually stuck together. That they don't quite match due to change in perspective and angle doesn't matter. People see immediately what you've done and appreciate it for the fun effect. (I have one such pair where the rocket went to about 6,200 feet ASL - BUT it was launched from 6,000 feet ASL ;-) - highest altitude water rocket shot in NZ (I think). Have the photographer stand back some way to do this. First shot shows water plume standing maybe 20 feet in air plus some sky above and second shot overlaps and has lots of sky and cloud and rocket high above. Chances of getting rocket in the first shot is about zero but water plume is easier. Rocket in second shot is at apogee so easy.

NB - be prepared to get you and your camera deluged.

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Added:

You could try "cheating a little" :-):

• Use a very low pressure so that the rocket comes off the off the pad more slowly, relatively. Adjust pressure to speed.

• Adjust fill %. Almost full and almost empty will produce less speed and less altitude.

• If you can adjust the orifice, use a smaller opening. Some launchers do not allow this directly - eg those that use a "launch tube". This can be overcome by inserting a movable orifice plate in th bottle that sits in the bottle during ascent up the rod and then moves into the neck once it clears the launch rod. Some place this on top of the launch rod so that the rocket collects it as it ascends. You need a restrictor that can be pressed through the rocket nozzle but which then expands / pops out / ... so it is trapped on ascent.

• Combinations of the above.

Answered by Russell McMahon

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