Von Karman nose cone Hx

Notes for making a NERO “H-x” fiberglass nose cone

Design

 

This particular nosecone is a Von Karman nose cone designed by applying the following formula:

  

r[cm] = 5*SQRT((ACOS(1-2*(x/25))-0.5*SIN(2*ACOS(1-2*(x/25))))/PI()

(Copy and paste from the excel file)

 

X = variable x, expressed in [cm]

pi() = pi = 3,1415 etc

ACOS = arccos = inverted cosinus

SQRT = root of

 

Length is 245mm, diameter is 100mm.

 

Von karman nose cone shape

 

Nose cone Hx – templates – A3 – 20.07.09 – JvdBe

 

 

BO has made a very convenient EXCEL sheet, please click here for the sheet. Small addition was made to “sheet 2 – colum 3”. These cells are calculating half the perimeter of the nose cone at various intervals. These values have been used to create an ACAD drawing / template to cut a single piece of fabric to cover one mold at once. 

 

Time schedule

 

It is important to take your time because you will only get one chance at it. The key to success is in the preparation.

 

Day 1

  • Apply 5 coats of mold release wax. 5x 15min = 1 hour 15 min.

Day 2 – I started right out of work and made the gel coat batch

  • 18:00 – Gel coat, 15min to apply plus 3 hours waiting to reach leather stage (things to do in the mean time: enjoy a beer, take up a new hobby or write an web page like I did)
  • 21:15 – Epoxy and fabric, 30min to apply plus 3 hours waiting to reach leather stage (more things to do: drink another beer (try not to get drunk), ponder about new rockets, getting fed up writing this webpage)
  • 0:45 – Trimming and bonding the molds = 30 min plus 3 hours to reach leather stage (actually I went to bed and set my alarm clock for this action)
  • 4:15 – Final trimming of the base = 10 min

Day 3

·         Mold separation and sanding of the flash

 

Total time to make a shell nose cone: 11 hours 30 minutes and counting.

 

 

Mould preparations

 

Before using a new mold, wax it with at least 5 coats. I use Meguiar’s Mirror glaze 88 Universal Mold Release Wax. Wax the 2 parts with a towel and the release was. After 2-3 min wipe the wax of with a clean towel. Between layers I typically wait about 15min. Allow to harden before applying gel coat.

 

Gel coat

 

I use POLY_POX GT 600 thixotrope epoxy resin together with POLY POX hardner 355 as a gel coat. You can add POLY-POX pigment paste to the gel coat to give it a colour. I use either white because it can be painted to any other colour or plain black for the NERO rockets.

 

To calculate the required gel coat I start from the basis that one layer of gel coat takes about 400gr/m2.

 

Diameter of the rocket / nose cone is 100mm. Length is +/- 250mm. Half of the max perimeter (½ D x π) +/- 150mm x length roughly results in the rounded up surface to cover (150 x 250mm, almost double the actual surface thus safety factor). The mold consists out of 2 parts so in total a surface of 2 x 150 x 250mm would need to be coated. This would results in 0,075m2 x 400 = 30gr gel coat. Totals: 20gr GT 600 & 10gr 355 harder. Add 5% (black) pigment pasta to the mix – 1,5gr, making a total of 31,5gr batch. Additional gel coat already taken into consideration by using almost double the surface area.

 

 

 

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Actual gel coat use on 03.03.09 was 19gr for both mould parts. I also raised the right / lower part of the mold by approximately 2cm to compensate for dripping out of the mold. Not much elevation is necessary. It takes approximately 15min to coat both molds and after that it will cure, at room temperature (20° C), in about 3 hours to “green” stage. This “green” stage is where the gel coat is sticky / tacky but doesn’t stick to your finger / leave a colour on your finger. Clean the edges with a towel right after coating the molds. Place on top of the heater to make sure it is at least 20° C. Use a brush with normal hair length to paint on the gel coat. Later cut the hair to get a tamping brush when fibreglass is added.

 

 

Fiberglass fabric

 

Total thickness of the nose cone is aimed to be around 1,0mm. The fabric I use is 160gr/m2 – 2X2 twill (keeperweefsel). One layer will add up to about 0,2mm so I’m using 5 layers of fabric to get a total of 1,0mm thickness. Note 19.03.2009:3 layers will probably works just as well and will ease cutting.

 

I used the template for a single piece of fabric as can be found here. The template has a 10mm overlap at the base of the nose cone. I only use 5 of that 10mm overlap and thereby basically shifting the form of the triangular shaped fabric 5mm towards the tip thus creating an overall overlap of a couple of millimetres. Actually this worked out great because I had set the length to 245mm but of course the distance from tip to the edge of the base travelling over the outside of the nose cone is more than 245mm. To much to think about “why it still worked”, probably due to the fact that the  fabric can be strechted / compressed to a certain extend (unintentionally I compensated my error during lay-up).

 

The pieces can be cut out with a sharp hobby knife. The base of the triangular piece is placed parallel / square to the weaves. Possible improvement is to check if placing it at 45° has some benefits (less unraffleling)? Cut from the base to the point works better not to unraffle.

 

 

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The epoxy that I use is POLY POX THV 500 in combination with the same POLY POX hardner 355 as used for the gel coat. Ten single pieces of fabric weigh about 46 gr in total. Trying to get a 50/50 glass / epoxy mixture I prepare a 75gr batch to have some spare epoxy for unforeseen touch ups. Totals: 50gr THV 500, 25gr 355 hardner.

 

To lay the fabric into the mold, first wet out the gel coat which by now should have reached leather stage, after that lightly fold the fabric in half along it longest side. Start at the center of the base with 5mm overlap and “tack” lightly in place, also tack the two sides /corners lightly in place and work your way along the centreline towards the tip, making sure the fabric is centred all the way (you can actually steer the fabric in place as your work towards the tip). Tamp with the brush the fabric into the epoxy and work out all the air bubbles. Add the second layer of fabric and add some smears of epoxy to completely wet out the fabric. Only after the previous layer is positioned into the mold to your full satisfaction, add the next layer of cloth. After wetting out the fabric, steering of the fabric can be done by carefully tamping the fabric into its direction (longitudinal or radial, both are possible). Continue until all five layers are fully soaked in epoxy and use as less epoxy as possible. 

 

 

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Actual use on 03.03.09 was 51gr epoxy, pretty damn close! Lay up of both molds took me about 30 min. Taka a chair, otherwise your back will be killing you while wetting out the fabric and make sure you don’t inhale the fumes. The two molds were placed directly on top of the heater (metal on metal contact) with the result that leather stage was reached after only two hours. Trimming of the overlap was not possible because the epoxy was quite tacky and flexible. After two and half hours it was more firm like rubber and not tacky but still flexible.

 

With a hobby knife the overlap was trimmed. To start, the knife was rasped from the outside against the fabric lengthwise. Back pressure was provided by a towel not to cut into your fingers. As soon as an opening was created the full length of the blade penetrated through the opening. With a cutting motion only to the outside (otherwise the lay-up will bent inward), with the blade pushed down to the mold and tilted a bit the overlap was trimmed of. The epoxy is still in green stage. Make sure that cutting is straight with the edge. Otherwise is will bend inwards when joining the two halves. Also check that it did not came loose at the edges, push gently back against the mold.

 

 

Bonding of the two molds, tip and base reinforcement

 

Sand the edges with 240grit paper so the edges do not stand out and the mold can be clamped together without any openings in which the epoxy can run. Use to glueclamps to secure the 2 halfs togethers.

 

To bond the two halves I cut 2 strips of AANPASSEN 245 x 40mm + 2 truncated triangular pieces 60 x 245 x 20mm thereby overlapping the first. All out of the 160gr/m2 twill. To add some thickness to the base I cut 3 strips of 40 x 300mm of 280gr/m2 plain weave fibreglass. The 280gr/m2 fabic adds about 0,4mm per layer so 3 layers will add 1,2mm to the already existing 1,0mm. Total thickness at the base 2,2mm. Or use 5 layers 160gr/m2 twill. This will probably give a bit more flexibility. After that I weigh 1,0 gr of 4,5mm chopped fibreglass strands for the tip. Weigh this in a cup to prevent a mess (they will get everywhere).

 

 

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The seven pieces weigh about 16gr + 1gr chopped strands. I prepare a 36gr batch to have some spare epoxy for unforeseen touch ups. Totals: 24gr THV 500, 12gr 355 hardner.

 

First run a smear of epoxy over the seam of the two halves and tamp it in a bit with the brush. It will automatically run down so add the epoxy at the base of the cone. Secondly place the 245x40mm strip along each seam and wet it out. Thirdly place the truncated triangular pieces overlapping the first strip and apply some extra epoxy if necessary. By now there is a small puddle of epoxy at the tip of the cone. Check if it would be enough to soak the chopped strand in. Add the chopped strands and tap them into the epoxy with the brush.

 

 

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Wet out the base and add the “heavy” plain weave. In this case, using your fingers to wet out the heavy fibreglass seemed to work better than the brush. To prevent the weave to unraffle at the base the use of paint tape might work (untried).

 

 

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Actual use on 04.03.09 was 24gr epoxy, the complete batch I made thus running out of epoxy, just enough. Always throw in the epoxy first and then the chopped strands. Sounds logical but I did it the other way round the first time. Again after it was set on the 20° C heater it took 3 hours to reach the rubber “green” stage at where the overlapping parts could be trimmed of. Next time keep at least 5mm of solid overlap as this makes trimming during green stage easier.

 

Release of the mold was easy because of the mold preparation. Place the halves horizontally with the lower part in a vice, use a plastic hammer and hit the top piece a couple of times at the sides. If all goes well you can wiggle the top piece right off.

 

Total weight of the nose cone is 136,1gr. Thickness at the base is 2,1 – 2,3mm, the math to calculate the thickness actually works.