Posts Tagged ‘fins’

 

Water Rocket Car MkXI – Launch Day

Even though it was a cold and slightly rainy winters day here, we were very keen to try out the Green Hornet with its new configuration, so we had a lunch day on Sunday afternoon.

The new rear end fins and low traction wheels were designed to make the water rocket car act more like a rocket for stability rather than a car, ie: The rear fins straighten the back of the rocket car. This means the rear fins need to overcome the friction of the rear wheels to effectively straighten the rear of the car.

Sounds good in theory .. time to try it out.

New Rear End of Water Rocket Car

Here is the video of the launch day. We managed only 2 launches due to the rain. We also had a father and his son on motorbikes stop to watch our second launch, that’s the noise you can hear in the background.

Results
Well we had partial success in that the launches were pretty straight, especially the first one, until there was any r/c steering input, as soon as the front steered, the rear of the car had no grip to follow and just fishtailed or did a 180 degree turn.  Obvious really after thinking about it, just like doing a handbrake turn in a normal car, …. On both runs we hit the gutter, we haven’t done this for a long time, even the r/c steering couldn’t control the cars direction when fishtailing.

Positives from the tests were the rear fins look to be working but we will need to put rubber back on the rear wheels, so we will see if there is enough force generated by the fins to straighten the car under thrust with the old wheels on.

The launcher updates worked great. The lightened chassis did work well as the car definitely accelerated quicker than the last revision.

We will keep working on it :)

Posted by on July 13th, 2010 Comments Off

Water Rocket Car MkXI – The Green Hornet

A lot of work has gone into reworking the rear of the rocket car and reducing weight as much as possible. The modifications include
– New rear chassis of the car
– Low friction (Lighter) rear wheels
– Rear wheels much further back from the nozzle – reduce thrust steering
– New Rear fin design – larger wing surface
– New nozzle alignment guide
– New electronics bay

The following parts were also modified to reduce weight
– Chassis
– Nosecone
– Rear wheels
– Rear Fins
– Electronics bay
– Rear Alignment guide

As this will probably be the last major change to this chassis before we build a new one, we also gave it a nice paint-job and christened it “The Green Hornet”

Rear of the Car
The Rear section of the chassis was replaced with a new design that was lighter, had the wheels much further behind the nozzle, allowed for the new 45 degree wing design as well as a space to try a new additional nozzle / air amplifier (not pictured yet). The rear section is only glued to the front section of the chassis with wood glue. It is extremely strong, as we found out when pulling off the older rear section of chassis which was also glued on.

Rear of the Car

Rear of the Car

The rear wheels have had the rubber removed to both lighten the rear wheels and reduce friction with the road. The theory is to minimize the force required to straighten the car, ie: move the rear of the car back into alignment with the front of the car. We are trying to make it behave more like a rocket than a car as per our previous down-force rear wing design which tried to reduce thrust streering by maximizing rear grip. Testing will tell if this configuration will work or not.

The nozzle alignment guide has been lightened but still ensures the nozzle points directly out the back of the car.

Rear Fins
The new rear fins are made of corrugated plastic to reduce weight. A set was made from thin plywood first but they were to heavy and too flexible, the corrugated plastic ones with the corrugations running from the centre to the tip were actually stronger and lighter. A 45 degree piece of wood was glued to the fin using PL Premium. The wood is screwed to the chassis to secure the fins.

Rear 45 degree Fins

Front of the Car
Changes here include a new electronics bay setup with a flip top section that opens up to allow access to the servo and steering rod connections. The on/off switch has also been moved to a better location now protected within the nosecone.

New Electronics Bay & R/C Steering

The nosecone has also been lightened and of course painted green :) A hoel to access the on/off switch

NoseCone

Nose Cone

Chassis Weight Reduction
The new chassis is 1.40m long but a lot lighter than the previous chassis.The jigsaw and hole saw were put to good use to lighten the chassis as much as practical while still retaining its strength. Being painted white, it is easy to see the holes cut out of the chassis. Triangles were used where possible for cross-members to retain strength.

Chassis in white

Front Alignment Guide
The front alignment guide between the first and second bottle was left as is. This is the section where the rocket itself is imparting most of the force to propel the rocket car forward, without it the rocket will shoot off and leave the car where it is. So it was thought to best leave this with a larger surface area rather than reduce it.

More Pics of the Car

View from the front

View from the Rear

Rear Wheels - No rubber

Weight Saving
The car pictured above weighs in at 2.560kg which, while considerably more than a water rocket, is far below the previous version of the car which weighed in at 4.5kg. This weight reduction should improve both the maximum speed of the car as well as the distance traveled .. hopefully.

Additional Work
The new “additional nozzle / air amplifier” will be a second nozzle behind the main nozzle. The idea is to try to push in some of the surrounding air into the exhaust plume using a difference in air pressure between the exhaust plume and the air rushing past the rocket car. This will sit in the area, behind the main nozzle and in between the rear fins. It will be removable so we can test the car with it and without it.

We also need to strap the bottles down to the chassis and do a hydrostatic test to make sure we have allowed enough space for the bottles to expand under pressure with the new alignment guide.

We need to modify the launcher to increase the length of the hose from the launcher to the female gardena connector, this is due to the extra length added to the car and the nozzle now being further away from the rear wheels

Posted by on May 20th, 2010 Comments Off

Slow Progress

Progress has been slow this last 2 weeks, or maybe the weeks just went past a lot quicker than usual :)

We have been working on building 3 wrockets to take to our next launch day. A one bottle wrocket a 2 bottle wrocket and a 3 bottle wrocket. I want all the wrocket to have at least 1 parachute. The single bottle will have a main chute module, the 2 bottle will have a main chute module as well as a drogue chute module and the three bottle will have a main chute module as well as a drogue chute module. Hopefully this way we can get some more testing done on the dual chute setup.

PL Glued Chute Structures
The additional wrockets have meant building additional modules which is in progress. These new corrugated cardboard structures have all been glued with PL Premium instead of my usual metal pins method.

I haven’t put the elastic bands on them yet for the pressure plate, but the sides seem fairly strong. Here are some pics

Main Chute Module - PL Premium

Main Chute Module - PL Premium


Main Chute Module (side)  - PL Premium

Main Chute Module (side) - PL Premium


Main Chute Module (inside)  - PL Premium

Main Chute Module (inside) - PL Premium


My PL gluing skills need a bit of work but the second one was better than the first one.
Here is a drogue chute module structure
Drogue Chute Module - PL Premium

Drogue Chute Module - PL Premium


Drogue Chute Module (side) - PL Premium

Drogue Chute Module (side) - PL Premium

Fin Fix
After the last launch day we had a fin come loose. I decided to remove the metal pins and glue the fins to the RFA. These seem very solid now. Ready to fly again.

Small RFA - PL Premium

Small RFA - PL Premium


Small RFA (Close Up of Fin) - PL Premium

Small RFA (Close Up of Fin) - PL Premium


The side attachments as well as the spine of the fin itself is glued to the bottle.

New Timer Mounting
As we have a new method of mounting the Tomy timers, we had to order some more, 5 arrived today .. so these will be put to good use in the chute modules with both the timer and the mount itself being used. The rest unfortunately, goes in the bin :(

5 New wind up Tomy Toys

5 New wind up Tomy Toys

Posted by on December 1st, 2008 1 Comment

Bigger Removable Fin Assembly

The rockets we are building are starting to get longer and heavier and so for 3 pressure bottle and higher wrockets we are probably going to need bigger fins to put the centre of pressure where we want it for stable flight.

We are very happy with the trapezoidal fin shape so we have scaled this design up approx 50% to make the fins based on a 12cm side instead of 8.5cm side. Also the length of the RFA body we have extended slightly so that the nozzle when attached to the rocket doesnt extend past the RFA body. This also makes it easier to stand up when not flying.

The build was very much the same as the previous RFA build procedure.  The fins were cut out in a trapezoidal shape

Bigger RFA Cutout

Bigger RFA Cutout

Notice the channels of the corrugated plastic are in the direction of the airflow when in flight. Leaving these open seems to work well.

Here are the three fins and the 6 support attachments. The support attachments are 10cm long and 8 channels wide.

RFA Parts

RFA Parts

Instead of using metal wire to attach the fins to the RFA body, this time we have used PL Premium glue. This is the water rocketeers glue of choice and was very easy to work with. We will see how it goes during flight but I am quite confident it wont come off. The RFA body was cleaned with Mineral Turpentine, then washed with soap and water to give a nice clean finish for the glue to adhere to.

Here is the finished RFA sitting on a bottle drying.

Bigger RFA finished

Bigger RFA finished

Bigger RFA Top View

Bigger RFA Top View

The unit weighs in at a hefty 45grams

Posted by on November 3rd, 2008 Comments Off