By Harry Advokaat
- An Egyptian creation story
- Location transmitter
- Delay element with ejection charge
- Motor casing
- Reload kit
- Pyro charges
- NLD23 launch day report
As a member of the NAVRO I had been working on our common clubprojects for some time, but then I felt the wish to build "my own" HPR rocket.
This is an account of my experiences. The way I did things are, by far, not the "ideal" way to build a rocket but luckily I had help of some experienced HPR builders (a.o. Pleun Punt, Remy van Herwijnen and Peter Leemker). I am pleased with the results. Recently the rocket had a fine flight on the NLD23 and I hope she will fly again several times! Maybe some beginners can use my experiences.
I named my rocket after Nut (say Noot), the Egyptian goddess of the sky. I think a rocket should not just be functional but should also be good looking and a good name is part of that. Work your fantasy!
As I am inexperienced with building rockets I ordered a kit, a Phobos 38 by PML, which you can get in Holland through Caveman Rocketry. I wanted to fly this rocket as "bare" as possible, so without electronics (just a location transmitter) and use a delay element and ejection charge for parachuting. This is also the method that the manual of PML describes. In hindsight this could maybe be not the best or smartest but it is usually the cheaper solution.
In the beginning I felt a bit confused about the names of all the motor parts. In the set there is a cardboard tube, called the motor tube. In this tube you can slide the motor. I used a reloadable AeroTech motor, which consists of a motor casing and a reload kit. The motor casing is an aluminium tube with two end closures and is reusable. Each flight you will need to buy a new reload kit. In this kit you'll find the propellant, O-rings, a nozzle, delay element and more.
According to the manual I first had to glue the "shock cord" to the motor tube.
Then, there is immediately an extra step that isn't in the manual. According to the manual you can put some masking tape on the casing to ensure a tight fit so that the motor doesn't come out during parachute deployment so you will have to buy a new one. It is better to make a "casing retainer". This can be a piece of wire or, like here, a nice aluminium plate. I filed this one out of a piece of aluminium plate. In this case it was necessary to glue some bolts on the motor tube before I could fit it in the rocket tube. To make space for the two bolts, I had to file some slots in the backwards ring. After this everything can be glued together and later I can screw the casing retainer on the end of the motor. Further down is another picture of the casing retainer in action.
Next, there is another thing not mentioned in the manual. In the manual assumes launch lugs (pieces of pipe that slide over the launch rod) are used for guidance of the first few meters of flight. Most of the HPR-clubs in Europe use rail guides that slide through a rail, usually the "Black Sky" type rail guides. The NAVRO uses bigger rail guides. Both types are attached with a M4 bolt onto the rocket. So I wanted M4 thread in my rocket and I used T-nuts (see the picture to the right).
First I folded the sharp points flat. Then I filed the threaded part a bit shorter because I only need the thickness of the rocket tube. Then, temporally, I glued the nut on a stick. To attach the nut to the inside of the rocket tube, I applied glue to the flange of the nut and insert the nut in the drilled hole from the inside of the tube. Then, quickly, I placed a bolt to hold everything while the glue hardens. Then I snapped off the stick. The result is a nice tread in the wall of my rocket. It is a bit of a fiddly job.
Now, it was time to place the fins. The construction of the Phobos rocket is very good because each fin is glued on six places: on the motor tube, on the inside of the rocket tube and the outside of the rocket tube.
It is hard to get the fins exactly 90 degrees on the rocket tube. Therefore I made a tool. I made a round hole in a small bit of wood, exactly the diameter of the rocket. Then I nailed some bits of wood on that hold the fins while the glue dries.
To apply the glue I used some kebab-sticks. The space between motor tube and rocket tube is just about 5mm. At last, it starts looking like a rocket!
Now, I got to do a bit of finishing. The glue fillets are never very smooth and you don't want to smoothen them out with your fingers. Therefore I added a layer of filler on top of the glue. This can smoothen easily and after hardening you can sand it. If you make a tool (a rounded piece of wood) you can make almost perfect, equal fillets.
After this, it is time to spray paint the rocket. First you have to spray a layer of primer before the paint will hold.
Here, I will confess two of my mistakes so that you can do things differently. First, I did my spray work on the tiles in my backyard. It is better to put some plastic on a piece of grass and spray there. If paint lands besides the plastic it will be gone after mowing the grass.
Secondly, I had my rocket fairly high up above the ground. The lower you have your rocket, the less paint will blow away and stain your surroundings.
Initially I wanted to use a H180W motor for my first flight. Now this is a 29mm motor and my rocket is made for 38mm motors so I had to make an adaptor. I wanted the forces of the motor to work on the rocket tube and not on glued inside elements so I made it like this:
The tube that holds the motor is from Caveman Rocketry. For the flanges I used very thin (3mm) MDF. You can cut this with a sharp knife or a saw. If you want to file a thin ring to size, you will have to support it with a piece of tube, it breaks easily.
First I cut out the centre, filed that to fit the motor tube, then cut the outside and filed that to fit the rocket. Then I glued the rings to the adapter.
My HPR-advisors weren't completely happy with my decision to parachute with an ejection charge. They say that the first launch everything goes well but at following launches problems can arise. The shock cord can burn a little when the ejection charge goes off. After some launches this could break and the your parachute will land without the motor section. To prevent this I was advised to put some glass fibre tube around the shock cord.
After inspecting the rocket (after the first launch) I wonder if this was really necessary. It is after all a few more grams so the rocket reached less altitude. At the other hand, it is for you to decide whether you go for safety or less weight.
A second problem with piston ejection is that residues of the ejection charge settles on the inside of the rocket tube. Because of this the piston could run less smooth during a second launch. The only remedy is cleaning well. After inspection I don't see so much dirt anymore.
Now it's time to consider the electronics. The NAVRO owns a R-DAS and wasn't flying a rocket themselves so I could use this one. That was very nice because now, I could measure the altitude. I decided to use the ejection charge rather than the ignition through the R-DAS as I had prepared everything for this now. Alternatively I would have to employ pyro charges.
I made a frame of pieces of L-shaped aluminium to support all the electronics. Some would prefer two pieces of thread but I think this system is easier. You can connect things with small bolts or rivets.
Make sure that the battery is supported at the bottom, it weighs only 50 grams but at a launch of 20G this becomes a kilogram! Some people use a battery clip to contact with the 9V battery. More experienced people say it's better to solder wires directly to the battery. During launch there's forces and vibrations that make a bad contact. An experiment with a beeper on a battery and rigorous shaking convinced me of the truth of that. So I soldered.
For the rest the electronics remained very simple: A switch in the + lead as an on/off switch leads the current directly to the R-DAS and the location transmitter. The parachuting outputs of the R-DAS are connected with two resistors of 1 kiloOhms, the breakwire input with a piece of wire.
In the rocket tube I made two holes, one to give access to a screwdriver that can operate the switch, both to let air in for the barometric measuring of height.
At this time the rocket was as good as ready. But I still have no reload. This meant that I couldn't launch in September 2005. More people had this problem and amongst other reasons this meant that the NLD22 was cancelled.
In April 2006 I had access to a H238T reload. This is a bit weaker motor than I wanted to use at first (a H180W) but if you can't do as you should do, you should do as you can do. One disadvantage is that this motor uses a shorter casing, 180mm instead of 240mm (luckily the NAVRO also owns this one) and this meant that I had to shorten the adapter because the hot gasses of the ejection charge would get in direct touch with the adapter. As I thought this was a shame (because later I'll be needing a longer adapter) I decided to line the inside with aluminium tape. This worked well, the cardboard didn't burn.
Now the end of the project is getting near I had Peter Leemker make me a trajectory calculation with Altipro (Altical, a slightly simpler version can be used on the NAVRO website). From this it appeared that my rocket was a little over stabile, it tends to lean into the wind. But, as my rocket with this motor is so fast (type cannonball) this was no problem. From the calculation I also learnt that the maximum height (515 meters) would be reached after 9,6 seconds. Now I had a little problem. My delay element that came with the reload was timed for 14 seconds. At that time my rocket would be falling for 5 seconds and have a high speed when my parachute deploys. A torn parachute is no fun. However, I didn't have the time to order a delay element of 10 seconds. When I was investigating this I found that it is possible to cut the delay element shorter with a knife. So I had to rely on this.
Another "too late" feeling I got when I realised I didn't have an igniter. It would be a shame if I had my rocket all ready to go and I couldn't light it. Luckily Robin Trap, a fellow HPR man, had some spare igniters and I could use one. I told you I am a beginner?
Something I could have done much sooner but that came as an afterthought was the logo on the rocket. First I wanted to use proper "decals" as they use in model building. In the end I used the easier way of printing a label and sticking it on.
On 27 and 28 April 2006 NLD23. On 27 April my rocket was approved to fly by the NAVRO and I loaded my motor (and shortened the delay element). The next day was for me the BIG day, my maiden flight!!! Luckily the flight seemed to go perfectly. The rocket landed without a scratch and will fly again a next time. A true Goddess of the sky. When studying the R-DAS data, I found there was one little flaw: The parachute deployed before the rocket was at its highest point. Apparently I shortened the delay element too much. You can see this best at the green line, the speed. After the big G-shock (ejection charge) the speed goes down very quick (an airbrake).
You can see several things by studying the graph. The black line shows the G-force on the rocket, The blue line the height and the green line the speed.
The motor worked indeed about one second (calculated 0,9 seconde) and produced a G-force of 20-15G's. It looks like Nut started off with 2G's but zooming in on the graph shows before launch it was a nice 1G. After the motor stopped propelling Nut had a good 0G. Then, at about 8 seconds, you get a positive G-force, which is the electronics capsule being pushed up by the ejection charge, right next a negative G-force because the parachute opened. Then the height goes down very evenly with about 10 meters per second until the rocket hits the ground. This gives an impact of -60G (my battery of 50 grams suddenly weighed 3kg). These sort of shocks are still very normal at hitting the ground. The G-force lasts very short so everything can handle it. At the ground, the normal 1G is restored. My altitude (the blue line) was maximal 475 meters, it could have been a bit more if the parachute didn't open prematurely. It seems that the Altipro calculation of 515 meters would have been spot-on.
To end this story, my plans. I have got the rocket flue for sure by now. Nut will fly again, maybe in September 2006 at NLD24. I hope she will have a stronger motor so then she will go up to about 900 meters, just below the need to use dual parachuting. At this time I will start building a new rocket, with dual parachuting and capable of going up to at least 1500 meters. Maybe to be launched in April 2007.
I did also think of a name: This will be Isis, the daughter of Nut. In mythology Isis was capable of flying over the country. For the future: Isis was the mother of Horus, the falcon god, so I am OK for at least three generations of rockets.
Until the next project!