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Date:  27th April 2009
Location: Workshop
Conditions: Pleasant
Team Members at Event: PK and GK

Acceleron V

We have been focusing on the development of Acceleron V over the past few weeks. Progress has been steady but good. We are now trying to get it finished by the next NSWRA launch day. We were going to be close for this weekend, but since the launch has been delayed at least a week we should now be able to finish it by then.

What's done:

• Base plate is complete
• Staging mechanism is complete
• All bottles have been spliced and have had their necks reinforced.
• Staging mechanism has been pressure tested to full operational pressure.
• Inter-bottle spacer rings have been made.

What's left to do:

• Both parachute deployment mechanisms
• All the wiring between flight computers, stager, pressure switch needs to be done.
• Mount cameras and altimeters in both booster and sustainer
• Run simulations and configure flight computers accordingly, and figure out how much water is needed.
• Top support bracket that holds the booster segments together. As of this weekend this is partly complete.
• Make and attach fins.
• Pressure test all the bottles to full operational pressure.
• Test the strength of the baseplate at full pressure.

We are still debating what to do about the guide rail, or if we will use one at all. Acceleron has always only used a pair of short ones.

The most amount of time has been spent putting together the baseplate and staging mechanisms. One of the goals was to reduce it's overall weight. Acceleron IV had the baseplate, support tube and staging mechanism weigh in at 635 grams which was essentially dead weight on the booster. The new baseplate and staging mechanism only weigh 372 grams so close to half the weight has been saved.

Baseplate

The baseplate is responsible for linking the three booster segments together as well as providing a common hold-down point for the entire booster. It supports the full weight of the sustainer during acceleration. We had to use aluminium for the main components due to the forces involved in this part of the rocket. The central nozzle (its not really a nozzle) needs to hold down around 80kg (175 lbs) when the rocket is fully pressurized. The entire nozzle (and rocket) is only held down by three stainless steel ball bearings in the brass release head. Plastic components here would not be able to hold that much force.

The central nozzle also supplies air to the sustainer via a non-return valve.

The horizontal brackets are also made of aluminium to provide enough strength for each booster segment. The diagonal struts are made from 6mm carbon fiber tubing and are anchored in PVC blocks near the nozzles of each booster. The central support pipe is made from 20mm PVC tubing. The struts are all epoxied in place. A plastic plug was made for the top of the PVC pipe that contained the epoxy while curing and provided a hole to let the sustainer air supply to pass through. The top of the plug helps to center the main PVC pipe leading to the staging mechanism.

After test assembling the base plate in the launcher with the actual booster segments fitted with their nozzles we discovered that the central mechanism just couldn't go down far enough to properly lock into the mechanism. Not wanting to machine up a whole new central nozzle, we decided to put a thick plastic washer between it and the base plate. Using longer screws to hold it all in place was enough to resolve the issue.

The other issue we had was while trying to fit the hose on the end of this central nozzle we found it a little difficult, so we decided to machine down the hose connector a little bit to make it easier. While machining the small thin tube caught on the knife and unceremoniously bent, and was damaged. While trying to straighten it, as suspected it snapped off. Dad came to the rescue and machined off what was left, then proceeded to cut a thread into the top of the nozzle and made a new separate hose connector component that simply screwed into the end. We epoxied it in place and all was good again.

Staging Mechanism

The staging mechanism is based on a modified plastic Gardena quick release connector. The normal hose connection has been removed and replaced with an aluminium connector machined to fit the thin tubing we are using. This has been epoxied into the central hole. The spring has also been removed from the collar. 3 screws are fitted to the collar to hook the springs on which pull the collar back. 

We are using springs on the collar this time rather than rubber bands to give more release force and also to prevent the problem with rubber perishing when it stays stretched for a long time.

The staging mechanism uses two servo motors for redundancy. They are separately connected to the staging mechanism's release arm by tiger tail (nylon coated braided steel wire) and crimped to prevent knots untying and strings stretching.

The servos are mounted inside the servo box to protect them from the spray. The steel wire is fed through a hole on the side.

A foam brace below the servo box is used to support the boosters as well as keeping the mechanism centered.

Non-return valve

The air supply to the sustainer passes through the central nozzle and through the non-return valve. The non-return valve is one adapted from the previous Acceleron rocket. Originally the non-return valve was going to be mounted inside the main PVC pipe, but mounting it on the outside meant we could more easily service it and check it for leaks. The air supply plastic tubing has a 3mm ID that we have tested to 180psi for 2 minutes. This tubing came from Clark Rubber.

Testing

We performed a couple of tests this weekend to check for leaks and if the servos can activate the staging mechanism at full pressure. Gardena connectors tend to tighten up under pressure. The springs had enough force to pull back the collar and release the sustainer. We used a small 600ml bottle completely filled with water as a dummy sustainer. We tested each of the flight computers separately connected to their respective servo motors. We were very happy when there were no leaks and the servos worked as expected.

Ring Support Brace

The top support brace holds the booster segments together while giving clearance for the sustainer fins to pass through it. It is being constructed from laminated balsa wood wrapped in fiberglass. We spent the weekend building the mold that is used for forming the balsa to the right shape. The mold simply consists of two disks of wood separated by a couple of blocks. These disks are then wrapped with a strip of sheet metal, and screwed to the disks. The balsa is simply bent around this and wrapped again by a second strip of sheet metal to form the other half of the mold. Tensioned wire holds the whole thing together.

We looked at quite a few woodworking websites on how to bend wood. The almost universal approach was using a steambox. There were plenty of details on how to easily make a steam box out of a length of PVC pipe.

We had a spare pipe on hand so we plugged the ends with cloth to prevent pressure build up. Fortunately we had a one of those steam cleaners on hand that generates lots of steam from a nozzle and so we inserted that through one end. The first attempt at making the steam box didn't quite go according to plan. After setting it all up and steam started building up the pipe became very hot and then softened up before drooping off the end of the table. I'm not so sure this was the right kind of PVC we were supposed to be using.

Attempt #2 was much more successful when we switched to using a piece of rectangular steel drain pipe. The wood softened up nicely and allowed itself to be bent around the mold. We are now waiting for the balsa to fully dry before gluing it together. When that is done it will be wrapped with a couple of layers of fiberglass to strengthen it further and waterproof it.