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Date:  10th June 2012
Location: Doonside, Australia
Conditions: 14C, overcast and mostly calm
Team Members at Event: PK, Paul K, John K, Jordan K and GK

It's been a while since the last update, but the last two launch windows had been cancelled due to high winds. This is a bit of a long write up so you may want to skip to the relevant sections.

• Improved Splices
• New Launcher
• ARB experiment

Improved Splices

This week we tried to improve on the splices we have been using for our water rockets. (Quite a while ago we tested a 1cm overlap splice without a sleeve that showed some promise.) We normally overlap the bottles by about 3 cm and join them with a 6cm wide sleeve so there are 3cm of PL premium on each side of the join. The amount of overlap required has always been only a bit of a guess. By reducing the amount of overlap between the bottles you reduce the weight of the splice due to less glue and a narrower sleeve, but you also gain more volume inside the spliced-pair.

The new splices still use the Asymmetric splice #5 technique, but the sleeve is only 3cm wide so you get a 1.5cm overlap. The narrower sleeve also makes it easier to apply the glue and assemble the splice. We still use Sikaflex 11FC to seal the splice.

Here is a comparison of the splices we have been using:

The new splice is 20% lighter than the symmetrical splices and 12% lighter than the asymmetric splice we had been using. The internal volume is also the same as the symmetrical splice. Using 3 of these spliced pairs in a rocket saves us about 30 grams compared to the previous asymmetric splice and about 50 grams compared to the symmetric splice.

We did a hydro burst test on one of the narrow splices to find out how much pressure it could actually hold. The bottles burst at 200psi which is about normal for the 1.25L bottles. It was a pretty big bang and I suspect there was quite a bit of air in it judging by the carnage. We do the burst tests inside an old scuba cylinder. The good news is that the splice did not fail which means we should have a comfortable safety margin. We will keep a close eye on these splices as we use them in rockets to see how well they hold up to regular handling and flights.

New Launcher

In order to perform this week's experiments we needed to build a new launcher. It is based on a classic Clark cable-tie launcher made from PVC piping. We made a thin walled aluminium tube to connect two pieces of PVC pipe with a small gap in between for the o-ring. This allows us to get maximum air flow through the launch tube. This o-ring setup is similar to this launcher:  http://cullytechnologies.com/demo/h2orockets/launcher_portable.php http://cullytechnologies.com/demo/h2orockets/images/portable_launcher/web/P5210028.jpg

We used two T intersections that allow two Air Reservoir Boosters (ARBs) to be connected. These are fitted with standard bottle threading so that we can either put normal bottle caps on disable the boosters, or attach the ARBs (bottles / spliced- pairs / spliced-quads). with standard tornado tubes. This allows us to compare the same launcher with and without ARBs.

Instead of using the cable ties, we just used a piece of PVC pipe with little screws in the end with flat round heads that hold the bottle down. The screws heads serve the same purpose as the tabs on the cable ties. We then cut slits down this tube to allow the individual segments to spring back much the same way we did for the Shadow launcher.

We added a quick-release adaptor to the bottom of the launcher to make it easy to fit our scuba hoses.

The release ring is just made out of two halves of  abent steel strip so that we could get the right size.

Experiment #1 - ARB performance

Aim

To investigate the performance difference between a launcher fitted with ARBs and one without.

Background and Theory

An Air Reservoir Booster (ARB) is another name given to additional volume added to a launcher in order to gain more performance from a launched rocket. ARBs have been around for many years in various forms. Here is a classic example used by Robert Youens in his Insane Air project. His launcher used a 30L ARB. They have also been popular on a lot of Thai launchers used for distance records. Here is a whole range of launchers that use ARBs in all sorts of configurations. In Thailand they call them "Tang Sam Rong". They have also been referred to as "Big Bang Boosters". Here are some forum discussions on ARBs as well:

http://groups.yahoo.com/group/water-rockets/message/7709
http://groups.yahoo.com/group/water-rockets/message/7514
http://www.wra2.org/forum/viewtopic.php?f=10&t=1871&p=12063

ARBs need to be used in conjunction with a launch tube in order to help the rocket. When a rocket is launched from a launch tube the volume inside the rocket increases as the launch tube is withdrawn and hence the pressure inside the rocket decreases by a proportionate amount. The volume increase is the volume of the cylinder made by the launch tube that has emerged from the rocket. Depending on the type of rocket this can either be a small or significant proportion of the total rocket volume.

The concept of the ARB is that as the rocket moves up the launch tube air flows from the ARB (which is at the initial launch pressure) to top up the rocket whose pressure is dropping due to the volume increase. By the time the rocket just leaves the launch tube there is higher pressure left in the rocket than without the boosters. 

A number of water rocket simulators can take this launcher volume into account in their simulations. For example see Dean Wheeler's simulator. Simulations indicate that you should be able to get a measurable performance boost with the ARBs.

We wanted to see how the simulated results compare with actual real world flights in a typical small rocket.

Experiment Setup

We built a small rocket for this experiment that consisted of a spliced pair of bottles tornado tube-coupled to a single bottle. We used our standard deployment mechanism for recovery. To evaluate performance we used the AltimeterOne altimeter to record peak altitude.

In order to get consistent launch parameters we did the following:

• Used the same rocket for all tests.
• Weighed the water that went into the rocket on a digital scale to give exactly 1000mL each time.
• On the first launch we set the target pressure on the regulator and left it on that setting for all subsequent launches. We opened the tank valve slowly each time to fill the rocket to the target pressure.
• We tried to alternate between launches with boosters and without in order to average out any changing weather conditions.
• We set the parachute deployment delay to 5 seconds to ensure that the rocket passed through apogee cleanly each time without a parachute. Time to apogee was predicted to be 4.3 seconds.

Test Rocket Parameters:

Results

The table below shows the results obtained from 9 test flights. All the flights were performed with the same rocket.

On the first test flight the rocket self launched without reaching the exact target pressure. So this flight couldn't be used for comparison. We lowered the launch pressure to 80psi in an attempt to prevent further self launches. The cause of the self launches was later traced to a problem with the release ring. Self launches again occurred on flight 5 and 9. We cannot use these flights reliably for comparison, though they were very close to the target launch pressure. After getting distracted, on flight 7 I forgot to turn on the altimeter so we couldn't use that one either.

Conclusion

The sample size is too small to give very accurate results. If we exclude the self launched flights as these would not have been at exactly the target pressure we get an average altitude of 306 feet (flights 2,4,8) without the booster and an average altitude of 317 feet (flights 3 and 6) with the booster. This is a difference of ~ 3.5%.

There are a number of environmental factors that also affect the measured altitude. The main one is cross-wind that may cause the rocket to weathercock and reach a lower altitude as it flies in an arc. Although on the day of the tests there was only a slight breeze, the rocket still landed at various distances from the launch pad indicating that some breeze may have played a role.

We entered the rocket and launcher parameters into Dean Wheeler's simulator to see how the predicted results varied from actual results.

The simulator predicted an altitude of 299 feet without the boosters and 325 feet with the boosters or an ~8% gain in performance. Which is the right ball park figure for the flights, though the unboosted flights went a little higher than predicted and the boosted flights were a little lower than predicted.

This experiment was done for a typical sized rocket, however, ARBs are likely to be of greater benefit to a narrow body rocket where the launch tube volume is a greater proportion of the rocket volume. For this test the launch tube volume is 266mL which is only about 8% of the total volume. More experiments are needed with bigger ARBs and narrower rockets.

The theoretical graph below compares the remaining pressure in the rocket as it is just about to leave the launch tube vs the proportion of the launch tube volume of the rocket's volume. The graph shows the values for a launcher with launch tube only (blue) and one with an ARB of equal volume to the rocket (yellow). The graph also shows a solid launch tube (pink) for comparison.

From the graph you can see that when the launch tube volume is a small proportion of the rocket volume the ARB doesn't provide much assistance, however, as the launch tube volume increases the ARB assistance starts becoming significant. I've also added a plot line for what a solid launch tube does to the internal pressure. I had always assumed that a solid and hollow launch tubes had identical performance, but this assumption was obviously incorrect. They behave similarly for small proportions of launch tube volume (as is common in normal rockets) but diverge as the launch tubes take up more space. Of course it is now plainly obvious once thought about in detail and plotted. It's good to learn from these things. :)

From the graph you can see that for this experiment, without the ARB the pressure left in the rocket was ~93% of launch pressure, and ~96% with the ARB.

Launch Day Events

The weather was overcast with a slight breeze, but not cold. We set up the launcher on the medium launcher's base and loaded the rocket. We used the full bore tornado tubes for this rocket so that the launch tube could fit through it. We filled the rocket with water and then laid it on its side so we could insert the launch tube without spilling the water and not getting too much into the launch tube. This was only partially successful and some water did enter the launch tube. After standing the rocket up we realised that the water that was in the upper bottle didn't want to drain to the lower bottle because of the tight fit between the launch tube and the bottle neck. So we pressurised the rocket to around 20psi which forced the water from the upper bottle around the launch tube to the lower bottle. This took about 30 seconds. Once the water was all in the lower bottle we could finish filling it and launch it.

The first flight self launched as we approached 100psi. Luckily the servo timer was armed and the altimeter was on, but it was a surprise as it made a loud pop as it launched. At first I thought something blew up, but it was just a typical full bore nozzle launch. :) The rocket landed safely. For all subsequent tests we dropped the pressure to 80 psi to try to avoid a self launch event again. The rocket self launched again on flight 5. We traced the problem down to the releasing ring slipping off. There is no spring to keep it up and because it is sitting on a conical surface it was forced down as pressure increased. We tightened the ring, but it still slipped again on flight 9. However, we were happy with the new launcher's performance. We will fix up the self release issue before the next test flights.

For the last launch of the day we made a longer rocket with 3 spliced pairs and a single bottle. One of the spliced pairs was a narrow splice. Initially when we put it on the pad with the water we had a really hard time getting the water to move to the lower bottles even at 40psi. The fit between the internal couplings and the launch tube were quite snug. As a result quite a bit of the water drained into the ARBs. We took the rocket off the pad, reamed out the offending bottle necks and put the rocket on the pad again without the ARBs this time. The reaming did the trick and allowed water to flow into the lower bottles much more easily.

It was launched at 100psi and the rocket took off well, but you could see a bit of a bend in the rocket under the high acceleration. The rocket flew to 460' (140m) feet and landed well.

All in all it was a great launch day with 10 good launches and 10 good parachute deployments. Now having launched our first full bore nozzles I have to say I still prefer the slower launches with the restricted nozzles.

Here is a highlights video from the day:

Flight Details