Aim

• To compare the difference in altitudes achieved using water-only and foam.

Experiment Setup

Location: Denzil Joyce Oval
Conditions:  20 degrees C, Easterly at 0-10km/h, cloudless sky.

The J4 IV rocket was used for the comparison flights. These are the rocket parameters:

	J4 IV readied for the first test flight. Note the crushing nosecone added for emergency landings.
	Flight data is evaluated in the field.

Images of the setup on location

Results

J4 IV was first off the pad with a couple of water-only missions. The rocket flew nice and straight and had nice deploys on both occasions.

Next we launched J4 IV three times with added foam and configured it to use Jet Foaming. The first flight pitched over quite a bit and the rocket powered through a long arc. Due to the non-vertical flight the altitude data could not be used for comparison. The last two foam flights were a lot more vertical and could be used.

The data was exported from the altimeter software and imported into Excel where the data was aligned in time and altitude offsets adjusted. The only difference between the flights was that bubble bath was added to the water for the foam flights. The pressure, capacity, nozzle size and weight all remained the same.

The following graph shows a comparison of the different flights.

Altitude comparison of water-only vs. foam

	And off it goes on one of the foam flights. Calm weather ensured mostly vertical flights.
	A water-only launch of J4 IV.
	Looks like there was a bit of a hiccup in foam production.
	A typical foam test launch.
	Some flights were very straight and foam produced was quite smooth.

Images of the foam and water-only flights

Video of the test flights

Flight Details

Conclusions / Analysis

• Water-only altitudes were 338 and 308 feet, although on video review of  the second water-only flight it didn't go quite as vertical so the average between the two is closer to 330 feet.
• The two valid foam flights were 316 and 326 feet and both were quite vertical. The average was 321 feet.
• This means that foam resulted in 2-3% less altitude.
• The simulation predicted altitude for water only was 320 feet (Taking a guess at the drag coefficient and nozzle efficiency values).
• This implies that foam has less performance, however, after the last foam launch I capped the rocket so the residue foam would not leak out in the car. Hours later when the residue foam condensed back into water I was surprised to find that there was approximately 80ml of water in the rocket. That is at least another 80 grams that had to be carried to that altitude.
  
  This amount does not include all the water that drained out of the rocket on the way down and when we carried it back to the launch pad. There may have been perhaps just as much or more water still left in the rocket when it reached apogee that did not contribute to the thrust as a reactive mass!
• This means that the foam powered rocket lofted another perhaps ~150 grams. This represents ~12% total water mass! Simulations predict a water-only rocket with 150 grams more should achieve only ~ 246 feet compared to 321' foam flight. (23% more)
• The question now is how do we make use of all that dead weight during the thrust phase?
• From previous thrust measurements we found that foam had 14% less total impulse, however, this only resulted in 2-3% less altitude. Most of this is likely due to the reduced drag on the rocket because it travels slower for longer.
• The noise in the altimeter data on descent is likely due to the parachute door flapping near the altimeter port hole.