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Date:   11^th March 2008
Location: Workshop
Conditions:  Pleasant.
Team Members at Event: GK and PK.

In this weeks update we describe the Polaron IV rocket and launcher in detail. The Polaron IV rocket was flown two weeks ago at the last NSWRA launch event. This rocket and launcher will be used in our next round of developments as we increase the capacity and strength of both the rocket and boosters.

Polaron IV Launcher

Design methodology

We wanted to build a flexible launcher for the next set of planned rockets, and in particular ones with boosters. As we look more seriously towards reinforcing bottles we wanted to make sure the launcher would be able to handle the higher pressures. One design criterion established early was that the launcher should operate up to 500psi. When dealing with design for higher pressures safety is paramount. We always use the correct fittings and properly rated materials. We are lucky in that dad has had close to 45 years of commercial experience in designing and building high pressure equipment for SCUBA divers, fire fighters, the military as well as experimental deep sea equipment. Any high-pressure components that we create in house always have a high safety margin.

We also realized that depending on rocket materials and configurations that for optimal results the main stage would need to operate at different pressures to the boosters. The launcher needed to be capable of launching both single nozzle rockets as well as clustered rockets.

Air Supply & Distribution

The air for the launcher is typically supplied in compressed form from a scuba tank via a commercial pressure regulator, but can be connected to an air compressor or a bicycle pump if really desperate. There are two quick release connectors on the side of the launcher. One supplies the air to the boosters and the other to the main stage.

We have now completed a separate self-contained control panel with all the pressure gauges, valves and bleed valves. We will describe this in more detail in future updates. The control panel connects to the two quick release fittings with two long hoses.

The main stage air supply line has a non-return valve fitted near the quick release fitting. This prevents water from draining from the rocket and entering the air supply hose.

Other than the flex hoses all other fittings on the launcher are brass, thick copper or steel. The flex hoses are made of rubber encased in a stainless steel mesh with stainless steel ends.

Diagram of the air supply components (Click to Enlarge)

Sliding Booster Nozzle Seats

The booster nozzle seats are free to slide horizontally. The reason for this is two-fold. It allows us to vary the diameter of both the main stage as well as the boosters depending on a particular rocket design. The nozzle seats just slide to the appropriate position to keep everything aligned.

They also slide a little by themselves as the rocket and boosters expand during inflation. This reduces the stress on the nozzles against the seats.

The nozzle seats have an integrated fill tube that doubles as a launch tube for the boosters. The fill tube fills the boosters with air only above the water line. Each of the fill tubes is connected together through the manifold below the launcher. This allows the air pressure to equalize between the booster segments. By keeping the fill tube inlets above the water line it prevents water from transferring between the booster segments.

Doing the pressure equalization on the launcher rather than the rocket itself allows the rocket to be lighter. It would also be quite difficult to separate the three boosters if they were connected to a single manifold on the rocket.

The nozzle seats and fill tubes are removable for easier packing and transportation. The nozzle seat is just machined from a section of brass and the aluminium fill tube is epoxied in.

The fill tube rather than being open at the top, is plugged and a small hole is drilled from the side just under the plug to let the air out. This was done simply to stop water entering the booster manifold while filling the boosters with water from the top and also during launch as the boosters clear the launch tubes.

Release Head

The release head is made from a brass 9mm Gardena mechanism. We chose a good quality one that uses ball bearings instead of the plastic tabs to retain the main stage nozzle. The mechanism is soldered to another brass section and is screwed down in the center of the launcher. The release head can be unscrewed and replaced with a different launch mechanism. The brass section directly under the release head has a bleed valve to allow the rocket to be emptied should a launch abort needs to be called. One problem with locating the bleed valve here is that you need to approach the pressurised rocket in order to release the pressure. This is a safety issue and will be resolved by allowing the bleed valve to be turned remotely using a string. The bleed valve cannot exist within the control panel as it needs to be on the same side of the non-return valve as the rocket. Putting the non-return valve in the control panel would allow too much water to drain into the hose.

A hose clamp holds two ends of a string to the sliding part of the release head. This string is fed through two pulleys at the base of the release head that turn the vertical motion into a horizontal one. The string is loosely threaded through a hole in a lever to allow the tension on both sides of the release head to be balanced.

Because the spring in the release head is quite strong, we use the lever to reduce the amount of force necessary to release the rocket without causing the launcher to tip over in the direction of the string being pulled.

Guide Rail

The guide rail consists of two U channels of aluminium attached so that they face each other with a 7mm gap between them. We made the guide rail 2m long because we expect the rocket length to increase in future experiments. It also gives the rocket plenty of time to get up to speed. The guide rail is removable for easy transportation. We went with the single guide rail as opposed to the three we have been using for simplicity. There isn't a whole lot of room on the launcher especially with the three boosters in the way. We went with this design as it is much stiffer than a launch rod that could easily bend under the loads of a higher-pressure launch. The disadvantage is that the rocket has to have guide rail lugs that add weight and drag.

You can ask questions or leave comments about the video HERE

 (If the video does not play, try the latest Flash player from Macromedia)

Polaron IV rocket

Payload Section

The payload section is composed of the inner support structure that holds all the components, and an outer aerodynamics shell with a parachute door and latch. The inner structure is made from corrugated plastic used for sign making. (Also known as Coroplast, Correx, Corriflute or Twinplast). We just use contact glue to join these together.

The inner support structure has the following components attached to it:

• V1.3.2 of our flight computer;
• A 6V battery pack made out of two CR123A lithium batteries;
• FlycamOne V2 video camera with 2Gb SD card;
• MOD4 Zlog altimeter;
• 8g micro RC servo; and
• And the spring loaded parachute deployment bay.

The outer aerodynamics shell is made from a 2 L bottle with the base and neck cut off. To get the nice elliptical nose shape, we glue in half a ping pong ball into the hole left by the removed bottle neck.

The inner structure simply slides into the outer shell and is retained by stops glued to the inside of the outer shell.

Body

The pressure chamber part of the rocket is made from 4 x 2 L bottles Robinson coupled together. The top bottle is turned neck up so that we can fill the rocket while it is on the launcher. This makes it a lot easier to align all the pins and tubes on the boosters without spilling the water from the bottom of the rocket.

The bases of the bottles are reinforced with a second layer jacket made from a 2.25L bottle and heat shrunk with PL premium glue sandwiched in between.

Removable Fin Section

The fin section of the rocket is removable. This section has all the fins with the booster retention tubes attached as well as the lower guide rail lug. The tapered part of this section ensures an even spread of force on the bottom of the pressure chamber. It also helps reinforce the lowest bottle. All the components are attached with PL premium.

Guide Rail Lugs

The guide rail lugs keep the rocket attached to the guide rail and help it steer in the correct direction as it accelerates and before the fins have a chance to start working. One lug is attached at the bottom of the rocket and the other is near the center of gravity of the rocket. The lugs are glued to short lengths of extruded PVC bar. This gives the attachment point some rigidity against the soft PET bottle.

Booster Retention Mechanism

The boosters are attached to the main stage with a simple pin type arrangement. The main stage has a pair of tubes (made out of ballpoint pens) glued to the surface. These tubes support the full weight of the main stage during the thrust phase of the boosters. The boosters have opposing pins (made out of coat-hanger wire) glued to their surface. A third pin is located near the top of each booster that is not load bearing but keeps the booster aligned with respect to the main-stage axis. This pin is offset to the side to allow the booster to sit flush up against the main stage.

While on the launch pad the boosters are free to move out of their nozzle seats. When the main stage is locked down in the release head, the tube/pin arrangements keep the boosters from flying off. When the main stage is released, the boosters produce more thrust than the main stage and that keeps them pushed up against the tubes and attached to the main stage. As soon as the boosters stop producing thrust, air pressure on their nosecones pushes them back and the pins slide out of the tubes.  

We favoured this arrangement rather than holding down all nozzles individually because it makes it much easier to synchronize the release of all of them. If they weren’t exactly synchronized, and one of the boosters let go even a fraction of a second later than the rest, its pins might slide out of the tubes in that instant, and you might end up with an out of control rocket. 

Polaron IV Rocket. Click to enlarge

Dummy Main-Stage Flight Tests

Two days before launching the real rocket we built a dummy main stage of equal weight and size to test to see if the whole system would work as expected. We did not want to use the actual rocket as the payload is quite expensive. The dummy main stage consisted of four 2 liter bottles joined together, but only the lowest 2 L bottle was pressurised. We wanted to include the approximate weight of water that would exist in the real rocket. We filled it up with 1.5L of water so there was only about 0.5L of air in the rocket. This was just enough to get the water out of the rocket in flight so that it would not be heavy on the way down.

We fitted the top of the main-stage with a retired NOAA nosecone and a parachute to help guide it gently back down. We also filled a zip-loc bag in the nosecone with about 150mL of water to provide extra ballast as the real rocket payload is a little heavier than the NOAA nosecone.

We launched the rocket at only 100psi as that was enough to prove the concept. The first flight was excellent and all systems worked as expected. The nosecone also came off at apogee and the rocket landed well.

The second flight was equally nice and straight, with the boosters separating at about the same time. On both flights we didn't notice any deviation from the straight path caused by uneven thrust. This was very encouraging. Although on the second flight the parachute of the main stage failed to open and the rocket disintegrated on the ground. The removable tail section and the guide rail lug further up the rocket survived without any damage and were fitted to the real main stage that evening.

Here is a video of those dummy main-stage test flights.

You can ask questions or leave comments about the video HERE

(If the video does not play, try the latest Flash player from Macromedia)

Setup and Launch Operations

The following list describes all the steps from launcher setup and rocket assembly at the launch site to launching the rocket.

1. Assemble launcher including fill tubes, and guide rail legs.
2. Connect air supply lines to control panel.
3. Apply silicone grease to the fill tubes and nozzle seats and the release head.
4. Apply silicone grease to the booster nozzles.
5. Seat all the boosters in the nozzle seats.
6. Apply silicone grease to the main stage nozzle.
7. Slide the main stage over the booster pins, and lock the main stage nozzle into the release head. (Verify all the booster pins are located inside the retaining tubes.)
8. Slide the guide rail in over the guide rail lugs and lock it in place.
9. Fill the boosters with water using a funnel.
10. Cap the boosters tightly.
11. Fill the main stage with water using a funnel. We use a long funnel that allows us to get past the first coupling so that water does not remain in the top bottle base.
12. Cap the main stage tightly.
13. Lock the nosecone/payload section in place.
14. Pack the parachute in the nosecone.
15. Configure and arm the flight computer.
16. Start the altimeter recording.
17. Start the video camera recording.
18. Clear the launch area of all personnel.
19. Pressurise the boosters to the required pressure, checking gauges on the panel for leaks.
20. Pressurise the main stage to the required pressure, checking gauges on the panel for leaks.
21. Verify that launch criteria are not violated. These include wind speed, people wondering close to rocket, or nearby aircraft.
22. Countdown and launch.

Flight Details