Monday, March 27, 2017

ELS / Parachutes

First the goodies. Check out this video to see the first system-level tests of the HAPP's parachute deployment system. Video analysis shows those chutes firing out at about 120 kph (75 mph). The video is a bit silly - gotta have fun while you work - but read on down below for the serious technical details.

The project is moving right along toward a summer mission flight. Flight controls are done. Structure is fully designed, parts have been machined, and final assembly is underway (details to follow soon). The next major system to validate is what the Apollo Program folks called the Earth Landing System or ELS, a.k.a. the parachutes.

From the rendering below you can get an idea of how this is supposed to work. Protruding from the upper deck of the HAPP are three mortar tubes which contain the parachutes. They're nestled between the vertical strakes or fins. During the mission, the HAPP will blow the chutes at a low altitude after free-falling from the edge of space. Most "space balloonists" use a chute that's always deployed. While that approach is simpler and less risky, those crafts can drift a large distance away from the launch site as they descend slowly through 30 Km of windy atmosphere while trailing a parachute.

Overall HAPP rendering

ELS shown in isolation

We're going for rapid descent. While the risks may seem high, consider the math. The HAPP weights only 9 kg and has an aeroshell that's 1 meter in diameter. In essence, the HAPP is a round wing. I calculate terminal velocity without chutes to be around 47 kph, or 30 mph for the Americans. If all three chutes fail, the craft will suffer damage on impact, but the data payload (4K, 360-degree virtual reality video on a micro SD card!) will most likely survive, and the HAPP is not going to do major damage to buildings, cars, or people.

If at least one of the chutes deploys from its mortar, the ground impact speed will be even lower. It doesn't even have to inflate - a tangled chute makes a fairly effective streamer. If all three chutes inflate successfully, the impact speed will be the same as dropping an object from about 30 cm in height (about 1 foot). Pretty cushy.

The chutes I'm using are Iris 60" Ultralight chutes from Gene Engelgau and the good folks at Fruity Chutes.

As for the deployment mechanism, I spent several weeks toying around with various concepts. I tried springs, elastic bands, and even a simple method that relied on the rushing wind surrounding the HAPP to pull the chutes out of their containers, assisted by small drogue chutes. None of those methods worked well. They were either too heavy, or did not eject the chutes with enough force, or both. The chutes need to be thrown forcefully and clear of the HAPP so the canopies have time to inflate before the wind carries them up and tangles all the lines together.

In the end I decided to get with the times and do a proper mashup of various technologies. For the deployment force, I adopted the approach of Cameron Tinder over at Tinder Rocketry. He has a system called the Raptor that was designed for model rockets. It uses redundant electronic igniters to light a black powder charge which in turn drives a sharp metal spike into a canister of compressed CO2. Once ruptured, the canister vents the CO2 into the parachute compartment and eventually forces out the parachute.

For the packaging, I went with the approach of the drone recovery system available here from Skycat. Skycat uses carbon fiber tubes to hold the parachutes. Chutes are deployed using a proprietary piston system, which is interesting, but the applicable model from Skycat is listed at 0.32 kg without a parachute. To get redundant systems and equip them with chutes would result in a heavier system than my approach. Plus, where's the fun in buying a finished system when you can waste weeks building your own?!

However, unlike the Raptor, I need to direct the gas into three separate mortars, so I designed a new manifold for the Raptor and had the awesome Jason Davis at Pioneer Cuts machine the manifold on his CNC mill. In the photo below, the manifold is the silver aluminum part that the CO2 canister screws into. It directs the gas to the mortars using high-pressure red macro lines and push-connects. My manifold mates to the charge and fire pin portion of a standard Raptor. In the photo, it's the red anodized aluminum part, the spring, and the small cylindrical steel parts in between.

A tech mashup: Part Raptor, part Skycat,
part home-brew, and 100% fun.

Let's walk through the details.

As the mortars pressurize with gas, the caps are pushed outward with significant force. Each of the caps are secured with four #2-56 nylon screws. These plastic screws are tiny, and eventually they shear off as the pressure accumulates. This allows the caps to fly off and the parachutes follow behind them. Getting all the math right was a fun exercise - what size of CO2 canister to use, what size of nylon pins, diameter and volume of the mortar tubes, etc.

For those who care, the theoretical maximum force required to shear four nylon pins is 603 N, which is equivalent to a 61 kg / 135 lb weight pressing on each of those three little mortar caps. The 45-gram CO2 cartridge I'm using, a Leland 87202, has enough gas to generate 3382 N on each cap should the pins prove stubborn or the caps get jammed, perhaps if the sealing tape around the rims freezes up at high altitudes. That's a safety factor of 5.6. I was shooting for 5, but cartridges only come in certain standard sizes.

I also put some one-way check valves in the macro lines. They're the black, blue, and silver cylinders located midway down the red macro lines. These valves allow gas into the mortar tubes but they don't let it flow out. This is critical, as the nylon pins are each a bit different (ah, the beauty of polymer chemistry), and they will shear at different times: In the video you can clearly see delays up to approximately 10 milliseconds. If one mortar opens first and there are no check valves, gas will leave the unopened mortars, flow back through the macro lines, and rush out of the open mortar. The unopened mortars will fail to deploy.

The mortar tubes are 9" sections of Size 18 INFINITubeV from Rockwest Composites. The caps utilize 1" sections of Size 17 tubes. The end plugs are machined circular sections of 2.2mm thickness carbon fiber plating. Plugs are bonded to the tube sections with this specialty 2K epoxy.

All in all, the total system weight including parachutes came in at 1006 grams. That's a little over 2 pounds for 5.4 square meters / 59 sq ft of total parachute canopy, explosively discharged, with redundancy for the chutes, the mortars, and the black powder igniters. Potential single points of failure are the CO2 canister and Raptor fire pin, but that's a risk I'm going to take.

Here's a picture of the chutes ready for folding and stowing in the mortar tubes.

Foosball table is optional for
successful parachute deployment

So that's about it for the Earth Landing System. Onward!

Thursday, March 2, 2017

Structure: Carbon fiber parts

Ever since I finally got the internal structure re-configured for optimal balance and aerodynamic stability using some serious 3D modeling, I have been working frantically like Doc Brown jacked on Red Bull to get the structural components built and assembled.

Soon I'll post a walk-through of the entire structure and the various components, but for now I thought I'd show you what the parts look like. This picture shows the whole suite of structural components, which are all fabricated from carbon fiber.

Need more fiber in your diet? Carbon fiber, that is.

The closed sections (round and square tubes) were purchased from Rockwest Composites and cut to length. The round tubes are mortars for the parachutes, and in the photo you can see one of the 60-inch parachutes peeking out of its mortar tube - stay tuned for excellent videos of explosive parachute deployment! The square tube is the main strut that runs vertically down the center of the HAPP.

The flat sections, also from Rockwest, were a bit more tricky. These are, variously, the internal decks (round), three aerodynamic strakes (triangular), and holders for the parachute mortars (various small pieces). I purchased rectangular plate stock of various thickness from Rockwest and had to machine the parts using a CNC mill. Thickness ranges from 0.7 to 2.9 mm, depending on the part. The largest single plate was 24x24 inches and 2.2 mm thick (SKU # 403-22). Not cheap!

Although I've received my checkout to run the large CNC flatbed router at Maker Works in Ann Arbor, Michigan, in the interest of time I had the awesome Jason Davis at Pioneer Cuts do most of the work. This is what the process looks like:

To save weight and maximize flight altitude, I had to abandon the aluminum mounting system described in a previous post. Now I'm bonding the structure together with specialty adhesive - in particular, this 2K epoxy from 3M. This approach means the craft will not be easy to repair if any parts are damaged in flight testing, but such are the engineering tradeoffs in aerospace!

It also means the assembly process is a bit more complex. To hold the parts together while the epoxy does its bonding magic, I decided to 3D-print some assembly jigs using high-accuracy SLA. Here's one of the jigs. This one holds the end caps onto the parachute mortar tubes. I made a total of 5 different jigs. Thanks as always to Steve and the great 3D printing team at ThingSmiths in Ann Arbor.

3D printed assembly jig on
end of parachute mortar tube

All together, what you see laid out on the table in the first photo is the entire structure of the HAPP. It weighs in at 1671 grams - not bad for a craft that's 1 meter in diameter, almost as tall, and will support a lot of serious hardware, including twin pressurized gas tanks, pneumatic valves, cameras, a custom-molded carbon/kevlar aeroshell, and a few other important bits (like the pyrotechnic-deployed parachutes!). All of which will be exceeding Mach 1 on a plunging descent through the atmosphere after flying to the edge of space...

Monday, January 30, 2017

Passing 35,000

Back in May 2016, I wrote a post describing the geographical composition of my audience as I crossed 10,000 readers. I had every intention of refreshing the data periodically, perhaps as I hit 25,000. Well, I should have checked the stats sooner, as not only did I blow by 25,000, but I should hit 40,000 in a few weeks!

The total today stands at 37,419. As I did back in May, allow me to give some color, followed by some awards to you, my dear readers...

For starters, here are some basic stats:

And here is the geographical breakdown:

Back in May I pointed out the curious (to me, anyway) fact that the highest source of readers outside the USA was Pakistan. Mystery solved: In the description of the blog for Google's search engine, I had included the keyword "bang bang." This is a reference to the particular type of control algorithm I embedded into the flight computer's autopilot function, explained here. It was an innocent but fateful mistake, as folks in Pakistan apparently search for popular Bangladeshi celebrities using the word "bang" followed by a name, and for whatever twisted reason harbored by its nefarious AI, Google was returning a link to the HAPP Blog amidst the search results. I know this to be true because Google shows me the search terms used by those who clicked through to the HAPP blog website!

For those curious, the #1 most searched-for Bangladeshi celebrity by those who also visited the HAPP blog is this lovely person:

Apu Biswas, Bangladeshi actress

Alas, with the "bang" reference deleted, the Pakistanis have fallen out of the Top 5 (although someone apparently has enough spare time to continue visiting the blog). The crown now belongs to France! Here's a special tribute to mark the occasion:

Vive la France!

Finally, I simply must give an honorable mention to a recent newcomer on the list. I have ZERO idea how folks in Oman found this blog, but my dear Omani readers - I salute you!

Monday, January 23, 2017

Cap'n, we need more power!

Now that I've started testing the field of view and placement of cameras - including the primary camera, the stonkin' Nikon KeyMission 360 - I've run right smack into an unfortunate reality: My old PC is useless for editing 4K video. It literally cannot run the editing software without immediately crashing. As the primary goal of the HAPP project is to get 360-degree 4K video and give my dear readers a virtual reality ride to space, this is a pretty big problem.

In addition, I'm having difficulties manipulating the full 3D model of the HAPP in AutoCAD Fusion 360. It crashes frequently and I've lost hours of work. For that matter, the flight control simulations in MATLAB/Simulink are painfully slow.

As I have lots of work remaining in all those areas, it's time to bite the bullet. I'm retiring the PC I built nearly 7 years ago (it was a banger back then!) and building a new rig this week that can handle these tasks.

For those curious about such things, here are the parts I just ordered today:

An 8-core CPU, a screaming graphics card, 64 GB of RAM, 12 TB of storage, and lots of liquid cooling ought to hold me for a while :-)

It sucks to deviate from the parachute-testing plans I had for the coming weekend, but this is a necessary step...

Saturday, January 21, 2017

Tether and camera mount

In the last post I described how I learned a hard lesson about taking time to do a thorough engineering analysis when necessary. In this post I'll apply that lesson to a small but critical part of the HAPP: The tether ring, which is where the HAPP is attached to the balloon that will lift it over 30Km into the sky. It turns out that this part is one of the most highly engineered components in the entire project.

From the start of the project I knew I'd need a robust place to attach the balloon. Initially I assumed this would be a simple metal ring of some sort. However, after completing the full 3D model of the HAPP, I decided to integrate two additional functions into the tether.

First, the tether will also serve as the attachment point for the parachutes that will deploy during descent. There's no sense adding additional mass for a completely separate structural element to achieve this function.

Second, the tether will also contain the mount for the primary camera, which is a 360-degree 4K ultra HD camera (specifically, the Nikon KeyMission 360). The only place where the camera will have a mostly unobstructed view of the world is from the top of the HAPP. If I attach the camera to, say, the side of the main strut, then a large portion of the view will be occluded. The camera therefore needs to go at the apex, but it can't interfere with the tether or the parachutes.

After several iterations using 3D printing to verify the geometry (thank you Owen and Steve at ThingSmiths!), I arrived at the following design:

In this rendering you can see a few important features. First, note that the base of the part will insert directly in to the square-section carbon fiber tube I'm using for the main body strut. The rendering shows four of the M3.5 mounting bolts that will go through the strut wall. The bolts are mainly to ensure position while I bond the metal body of the tether to the carbon fiber strut using some specialty adhesive.

One side of the camera - and one of its lenses - is visible in the rendering. This camera is symmetrical and has a second 180-degree fisheye lens on the other side. The camera automatically stitches together the two 180-degree videos to form a 360-degree virtual reality video in all its glory.

You can also see two of the three parachute attachment rings at the ends of short arms that jut out horizontally. There is the main ring enclosing the camera, and finally there is the actual balloon tether ring sitting on top. It's not obvious from the picture, but the parachute rings and camera enclosure are designed to be almost invisible to the camera's field of view. Here's a quick demonstration. In this 360 video, note the kevlar/carbon fiber lower aero shield that I spent the entire summer of 2016 learning to fabricate...

360 camera field of view confirmation.
If this doesn't play on your screen in 360,
try viewing with YouTube here.

What you can't see is how the camera is attached. There's a 1/4"-20 UNC bolt going up into the camera's standard mounting socket. However, as the part will be bonded to the main strut, the bolt must be started and tightened using a long tool inserted from the bottom of the 836 millimeter-long main strut. To facilitate this blind insertion, I added a "docking cone" to the bottom of the part, which is visible below:

Bottom view of tether / camera assembly
showing blind insertion cone for 1/4" bolt.

With the basic geometry defined, it was time to optimize the part for light weight while insuring it can withstand the forces resulting from opening shock of the parachutes. A skinny part is better as it minimizes total weight and maximizes ultimate flight altitude, but more importantly, this part is at the apex of the HAPP and will strongly contribute to a higher center of gravity. This is bad - see my struggles with CG in the last post. Too skinny, however, and the part will fail when the chutes deploy. How to find the right balance?

For this we need some real engineering, and the right tool is finite element analysis. The 3D modeling package I'm using, AutoCAD Fusion 360, happens to have a nice little FEA component that's extremely intuitive and easy to use. To run the analysis, however, we need to know what maximum force we can expect from the parachute opening shock.

I performed the shock analysis by hand using some references from NASA back in the 1960s (the technical gift that keeps on giving!), the Naval Ordnance Lab, and the Naval Surface Weapons Center. I used numerical integration to scale time/force curves given known parameters, yielding a conservative maximum force estimate of 1150 newtons.

If you want to play around with such things and don't feel like rolling your own, I recommend you check out a little piece of software called OSCALC. It gave a worst-case force of 1050 newtons, thereby validating the accuracy and conservative nature of my approach. It also comes with a user manual that walks through some of the math.

The analysis assumes we are using three of these lovely parachutes from the folks at Fruity Chutes. The number and size of chutes was selected to yield a landing speed of 2 meters per second, which is equivalent to being dropped from a height of 0.2 meters. If one chute fails to open, the landing speed will be only 3 m/s, which is still pretty cushy.

With the forces defined, it was time to optimize the design using FEA. I started with a design based on manual calculations using cantilever beam equations. From there, I went through 7 cycles of fine-tuning the design and the choice of materials. Finally I settled on aluminum 7075-T6 and the design shown in the renderings above.

Here's a little video showing the final design after optimization. The amount of deformation is greatly exaggerated - in real life you'd not be able to see it with your eyes. The blue color denotes a high safety factor, and yellow, orange, and red colors denote progressively lower ones. A safety factor of zero means the part is right at the yield strength for the material. I optimized the design so the region with the lowest safety factor is at 0.5 - appropriate given the conservative analysis of parachute forces.

Analysis of safety factors using finite element analysis.
The short video show how the arms deform as
the parachutes deploy.

Same analysis, just a snapshot instead of the video

To give you an idea of how the design evolved with the FEA analysis, here's a comparison of the parachute attachment arms from version 1 and the final version 7. The arms got shorter, they changed from a round profile to a variable elliptical profile, and the chute attachment rings got substantially smaller. All of that extra metal was not necessary. Besides changes to the arms, the base portion that inserts into the main body strut got longer. This was mainly to accommodate the cone-shaped feature that assists with blind insertion of the camera mounting bolt.

Evolution of parachute arm
from v1 to v7 using FEA

Final part weight is 139 grams. Currently, the part is being CNC milled out of a single piece of 7075-T6 aluminum by the outstanding team at ProtoLabs.

OK kiddies, this concludes our case study in mechanical engineering for the day. See you next post for some live testing of parachutes!

Sunday, January 8, 2017

Center of gravity

Happy New Year, and Happy 1st Anniversary of the HAPP project! It's been a whole year since I decided to undertake this project. It might also seem to have be a year since my last blog update...

There is a reason for my lack of blogging. Well, two reasons actually. The first reason is that I got a new day job in mid-July, and I've been pretty focused on the new gig. Gotta pay the bills for the HAPP project!

The second and more crucial reason is that I hit a roadblock as I started to assembly all the flight hardware, especially the structure. Turns out I had balance issues. Specifically, I couldn't figure out how to package the guts of the HAPP such that the center of gravity was sufficiently low. The CG must be lower than the aerodynamic center of pressure if the HAPP is to remain aerodynamically stable during the free-fall portion of its descent from balloon burst down to the altitude for parachute deployment. If the CG is too high, the HAPP gets tippy.

Think of a shuttlecock - most of the weight is at one end, the nose, and the center of pressure (where the wind catches it) is at the other end, the feathers. This make it stable as it flies. We're trying to achieve something similar with the HAPP, but we want it to be stable with the lower shield facing downward - you may recall that the HAPP is essentially a scale replica of an Apollo Command Module.

The "final" design I've been using was based on some internal packaging that evolved from multiple prototypes, and it relied on some rough hand calculations for the center of gravity. I thought I could roughly calculate the CG accurately enough that some final adjustments after building the main structure would be sufficient to fine-tune the CG. Not so! After weeks of staring at a lab table full of parts, I just couldn't make it work.

To find the solution, I had two options. Option 1 was to keep buying lots of parts and try, try again until I found the answer. This is OK when dealing with plywood and cardboard prototypes. This is not OK when dealing with flight hardware that's primarily carbon fiber - the stuff is expensive!

Option 2 was to model every single part in 3D software and play with the design until I figured it out. This option is cheaper, but requires time to model all the bits and pieces. Every screw. Every panel. Every piece of electronics. Every pneumatic component. All of it.

Having already spent too much money on this project, I went with Option 2. I used AutoCAD Fusion 360, which is an excellent free program. It took some weeks, but I finished the modeling, and I found my solution to the CG issue. It entailed a serious re-arrangement of the HAPP's guts. As a bonus, I was able to cut almost 1 kilogram of weight out of the design. That means higher potential maximum altitudes on flight day.

With the design completed, I have started ordering parts and fabricating the final structure (no really, it's final this time!). In the coming weeks I'll post some photos and videos of the build process. In the meantime, here's a rendering of the section between the lowest deck (the tank deck) and the second deck (the propulsion deck, which supports the pneumatic valves). I omitted the pneumatic hoses and some wiring for clarity. Two of the three canisters holding the main parachutes are just visible at the top of the image.

With the CG roadblock cleared, the project is back on track, and with a hard lesson learned. Sometimes there is no substitute for thorough, quantitative design work before you build!

Valves and low-pressure regulator hanging
down from the Propulsion Deck.
Tanks visible at bottom.

Bonus pic: 3D-printed thruster pack and
quick-connects for air hoses, mounted
on the end of the jet arm

Sunday, September 18, 2016

Aeroshell Finale: Part

If you've stuck with me this far, you're familiar with the overall strategy for fabricating the HAPP's outer aeroshell: Make the plug, then the mold, and finally the part. Well, after perhaps 250 hours of study, experimentation, failures, and troubleshooting, here we are at last at the part-making stage. Let's get busy.

First, let's start with the end in mind. Below is a picture of the part we're making - the lower aero shell. Recall that the HAPP is essentially a scale model of the Apollo Command Module. The lower aero shell is analogous to the lower heat shield on the Command Module. Of course, the HAPP will not experience the blow-torch heat that the CM felt upon reentry at Mach 32, but we desire aerodynamic stability at supersonic and trans-sonic speeds, and the Apollo design achieves this. (Interesting technical footnote: Blunt shapes experience less heating upon reentry compared with pointy shapes. This was a military secret until NACA, the predecessor of NASA, finally published it in 1958. Read it here.)

If you just came here for a sexy picture of the finished product then you're done. See you next post. If you care to see how the sausage is made then proceed!

Finished lower aeroshell.
Mold parting line is visible and will
be sanded out and polished.
This part is 1 meter in diameter.

Lookin' like a BOSS with that Kevlar/carbon
fiber weave on the A-surface

OK so here we go. Below are two pics from my first attempt at the full-scale finished part. The left frame shows the beginning of resin infusion - recall that we're using the vacuum infusion process I described previously. The portion that's been wetted by resin is darker than the surrounding material.

The right frame shows the "finished" part - a total disaster! The resin did not thoroughly infuse around the part, and there was a lot of raw fiber that I had to cut away. You can still see loose fiber around the edges. In addition, the layup consisted of only two layers of fabric, and it did not provide sufficient rigidity. The part is quite floppy and feels a bit like leather. Very expensive and useless carbon fiber leather, that is...

Trial #1: Total failure!

So onward to trial #2 with lessons learned.

The first process improvement involved the PVA. Recall that this is a non-stick coating sprayed onto the mold surface so the layup and resin don't adhere to the mold. In trial #1 the PVA pooled in the bottom of the mold. In trial #2 I drilled a drain hole and let the excess PVA flow out. This resulted in a highly uniform coating inside the mold.

PVA drip hole and uniformly-coated mold.
Mold legs were amputated to facilitate
covering the entire mold with the vacuum bag.
Easy come, easy go.

Next came the layup. The main improvement over trial #1 was to add a third layer of fiber and an intermediate layer of Lantor Soric foam core. This improved layup was developed in the previous post.

Clockwise from top left:
Kevlar A-surface layer;
3K carbon twill;
Lantor Soric foam core;
3K carbon plain weave B-surface.

Here's a view of the first three layers. I trimmed the excess overhanging material before bagging and infusion.

Working on the layup

For the outlet port I wrapped a spiral plastic tube with permeable nylon peel ply, ran it around the lip of the mold, and connected it to the vacuum pump. This provided an even vacuum draw all around the part.

Nylon-wrapped spiral tubing (green) on mold lip.

Next I placed the entire mold in a vacuum bag and affixed the inlet ports. For the bagging material I used a very elastic sheeting called Stretchlon 200. And below you can see the final major improvement versus trial #1 - the addition of more inlet ports. This allowed for faster resin flow and more even coverage around the part.

4 resin inlet ports

At last it was time to infuse. After activating the vacuum pump I opened the resin valves and established an even flow. Below is a short video that gives you a sense of the flow speed. This is 3/4" outer diameter plastic tubing. In the future I will eliminate the bubbles by de-gassing the resin in a vacuum tank prior to infusion. For this first aeroshield, which will be sacrificed in flight testing, the bubbles won't affect the final part significantly.

With the additional resin inlet ports, infusion progressed evenly and quickly.

Infusion progress

I also discovered that the infusion ports needed constant support. With the help of a friend (thanks John!) we quickly improvised the support "lattice" shown below. In future builds we'll use an actual lattice.

Resin feed tubes clamped in place

Infusion: Done. Resin cure: Done. Now it was time to decant the mold from the vacuum bag. It was a little tedious and some helping hands made for fast work.

Decanting from the vacuum bag.
Chunks of red flow media are visible.

Below is the decanted part and a close up of the B-surface. Remember that the B-surface is not normally visible and its appearance is not critical. Nevertheless, the surface doesn't look too bad. You can clearly see the imprint of the red flow media that sat on top of the layup.

B-surface detail

Finally, after a couple of hundred hours of learning and failures, it was time for the coup de grace: De-molding of the finished part. That's me in the left frame below with a big smile on my face because the part looks good. John is holding the part in the center frame with half of the mold removed. The right frame shows the part and A-surface in all its Kevlar glory.

De-molding the part

This picture says it all. Success!

Thanks for helping, John!

Here's a view of the part being rotated so you can get a better sense of the shape and dimensions. There are a few tasks for later - Trim the edges, fill in a few minor defects, and polish the outer A-surface.

And here's some of the A-surface detail. Kevlar, beautiful Kevlar... the mold parting line is clearly visible in this picture. I will sand it out and polish later.

A-surface detail

Here's the lower shield in the approximate position it will maintain on the HAPP. I'll do a post about the mounting system later. There is an upper aeroshell as well - pics to follow soon.

The lower shell goes here

And once again, here's the finished part. I essentially spent the entire summer learning to fabricate an aerospace-quality composite aeroshield. It was far more work than I expected, but it was a fantastic learning experience and the part turned out better than I expected.

Aeroshell... beast mode

During this final trial (#2) I noted many process refinements for future production runs. My friend John provided many of the more useful observations and ideas. There are also a few small quality issues with the part that need to be ironed out but the process refinements should fix them.

Undoubtedly I will make a few more parts in the future as we test & destroy before the first mission flight. With some luck, the process will only get easier and the parts will be even higher quality.


Aeroshell Part 3: Composite design

With the plug and mold fabricated, the next step is to figure out what type of composite material to use. This is an entire field of engineering unto itself. Composite materials typically consist of a resin matrix surrounding some sort of fiber, usually carbon, fiberglass, or Kevlar.

There are a variety of resins available and they generally fall into one of three categories: Epoxy, polyester, or vinyl ester resins. Each has pros and cons, but for lightweight aerospace parts, epoxy is a good choice. After playing around with all three resin types, I settled on an epoxy called CLR from Rock West Composites. It has attractive mechanical properties and it contains a UV inhibitor that protects the Kevlar fibers we're going to use from harsh sunlight at high altitudes. It also cures to a nearly clear appearance rather than the yellowish tint found with many other resins. If we're going to do this thing, we might as make it look good - don't want to hide those beautiful carbon and Kevlar fibers!

As for the fibers, I decided to use carbon for its strength-to-weight properties, but I also wanted to add some impact protection to the outer layer of the bottom aero shield. The parachutes should bring it down slowly enough, but there's always the chance of landing on a sharp rock. To achieve this impact protection, I decided to use a kevlar-carbon fiber blended weave.

Kevlar makes a nice outer layer because the shell is convex. This means the shell's outer layer will be in tension when it impacts the ground. Kevlar is extremely strong in tension, but doesn't like compression as much when used in a composite structure. Therefore, the inner layers should be carbon fiber. The remaining question is: How many layers of fiber are necessary to reinforce the Kevlar and give acceptable strength for minimum weight?

After a few rough calculations it was time to experiment. I made several small samples with various numbers of carbon fiber layers and intermediate reinforcing materials. The process I used is called vacuum infusion and it's the same type of process I will use to make the final aeroshield. Alternative methods are hand-layup with vacuum bagging and pre-pregnated fiber with an autoclave. Vacuum infusion can almost achieve the low resin weight content of pre-preg (around 40%) but without the added complexity and expense of having to use an autoclave.

In short, vacuum infusion entails placing the layup - the stack of fiber materials - into an airtight plastic bag. At one end of the bag is an inlet port. At the other end of the bag is an outlet port. The inlet port is connected via plastic tubing to a pot of liquid resin. The outlet port is connected to a vacuum pump. When the pump is activated, resin is drawn into the layup and infuses thoroughly throughout all the fiber layers. Because the interior of the bag is a vacuum and the exterior is exposed to the atmosphere, the layup experiences 1 atmosphere of pressure (14.7 PSI, 101KPa) and the layup is compressed strongly. Because of this, the layup does not retain excess resin. The excess is drawn out of the outlet port and into a trap for disposal.

Carbon fiber layup ready for vacuum infusion.
Inlet (resin) and outlet (vacuum) ports are visible.

Contents of the vacuum bag:
Stiff backing board (white).
Carbon fiber (black).
Peel ply (green) - removed after infusion.
Flow media (red) - removed after.
Absorbent batting (white) - removed after.
Vacuum bag (purple).
Tacky tape (grey) - to seal the bag.

The main decision is how many layers of fiber are necessary. Below you can see some samples I made. The left-most sample is just a raw piece of Kevlar fabric with no resin to show the contrast in appearance versus pure carbon fiber. The other samples have 1, 2, or 3 layers of fiber, and some also have a foam core called Lantor Soric

Various layup combinations for testing

After testing the stiffness and strength of these layup samples, I selected a layup consisting of Kevlar/carbon weave for the outer layer, a 3K carbon fiber twill, Lantor Soric, and a 3K carbon fiber plain weave for the inner layer. The outer layer is the "A surface" (to use a term from the automotive industry) and will be visible. It needs, therefore, to look awesome! The inner layer is the "B surface" and the final finish and appearance are not critical.

This layup might be overly-strong for the HAPP lower shield. If so, we might be able to eliminate one of the layers (probably the Lantor Soric) and further reduce the weight of the part.

Let's make one and see!

Monday, September 12, 2016

Aeroshell Part 2: Mold

Recall from the last post our 3-phase process for fabricating an aerospace-quality, carbon fiber aeroshell to serve as the "skin" of the HAPP:

  1. Make a scale model of the aeroshell. This is known as a plug.
  2. Using the plug, make a fiberglass mold that's a negative image of the model.
  3. Using the mold, form the various layers of carbon fiber and other materials to create the actual part that will fly on the HAPP.

Last time we wound up with a nice, smooth, and shiny plug, finished off with a glassy epoxy coating. In this post I'll take you through creation of the mold. I found the instructional videos and reading material at Fibre Glast, Rock West Composites, and Easy Composites to be especially useful.

And quick props to the guys at Fibre Glast - I purchased most of the materials for the final mold and parts from them after trying other suppliers for various prototypes. Fibre Glast has super fast turnaround and their products are great. Thanks guys!

Shiny plug, ready to create the mold

We'll create the mold in two halves so we can open it up to extract the plug, and later, the parts.

The first step is to add some temporary barrier features around the plug using white plastic corrugated signboard. One barrier goes across the plug diameter on the outer surface. This will allow forming of a fiberglass lip to mate with the other half of the mold. Another barrier goes around the plug and sits flush on the mounting board. This will allow forming of a fiberglass flange that will be quite useful once we start forming actual parts. (Spoiler: The flange is for attaching a vacuum bag with tacky tape. Don't worry, we'll get to that later.)

Here's the plug with barriers affixed using pink styrofoam and hot glue.

First half of mold to be formed on side
opposite from the pink styrofoam

The second step is to prepare the plug surface. Why, you ask, do we need to do any more than the tedious repetitions of coating, filling, and sanding that we performed already? Because the resin we'll use to make the mold will bond with the plug's grey gel coat unless we first slather it with appropriate release agents.

Here we'll use 4 coats of mold release wax with an hour of drying time between the second and third coats. Then we'll spray it with a thick layer of polyvinyl alcohol (PVA) release agent and let it dry for a few hours. PVA is a water-soluble (but not resin-soluble!) chemical that dries to form a thin green film between the plug and mold. This film can be easily peeled off when we're done. Here's the plug with PVA sprayed on one side, just prior to creation of the first half of the mold:

Green slime time with PVA. The beige
balls of clay are registration dots that
will form features to seat with the other
half of the mold.

Now it's time to fabricate. First I paint the PVA with orange tooling gel coat, similar to the grey gel primer I used on the plug. The orange gel coat will ensure a smooth surface on the mold face as I build up the structural layers of fiberglass behind it. The orange color also helps to show surface imperfections so the mold can be properly conditioned. Plus it looks kinda cool.

When the orange gel coat starts to cure and get tacky (I used 2% MEKP catalyst) then it's time to start laying up some glass. The first layer is a 2-ounce plain weave glass. This fabric prevents print-through of the heavier layers behind it. The next two layers are 20-ounce tooling fabric. I wet all the layers with isophthalic polyester resin using a paint brush and small roller to work out the air bubbles. After curing, the first half of the mold looked like this.

Tilted for a reason....

You may notice I didn't take any particular care to create an aesthetic pattern with the fabric layup - I was only concerned with functionality and strength. You may also notice the base board is inclined about 45 degrees. This is to ensure the wet fiberglass drapes down around the outer edge of the plug using the force of gravity. Otherwise the glass would droop away from what would otherwise be an undercut radius.

At last it's time to remove the mold and see what we've accomplished. Here's the first half of the mold immediately after coming off the plug. I've peeled away the layer of PVA and I'm starting to trim up the edges with a Dremel.

Doesn't look so pretty now but just wait!

With the first half cleaned up, it goes right back onto the plug, which is now tilted in the other direction to create the second half. We repeat the same process - wax, PVA, orange gel coat, 2-ounce fiberglass, then two layers of 20-ounce fiberglass. Allow an overnight cure.

Wax/PVA, orange gel coat, fiberglass

Now for the finishing touches. After cleaning up the second half of the mold, I assemble the two halves together using some #10 bolts. I also fiberglass on some legs to stabilize the mold as I fabricate parts.

Bolts and legs

Finally, I fill in the small gap between the mold halves with automotive body filler, and finish the entire surface by sanding with a progression of #120 to #2000 grit paper. As a finishing touch I buffed the surface with automotive rubbing compound.

The finished mold surface. Ready for parts.
The outside is ugly but the A-surface is pristine.
It has it where it counts!

Whew! Another 50 hours at least. Now to figure out what kind of carbon fiber composites to use... next post!

Sunday, September 11, 2016

Aeroshell Part 1: Plug

It's been a long hot summer, and it may seem from the lack of blog updates that the project has petered out. Not true! I've spent the summer learning how to fabricate aerospace-quality custom carbon fiber shells to use as the aerodynamic body of the HAPP. This is the "skin" that will cover the carbon fiber "skeleton" I developed earlier.

It took a lot of trial and error and a lot more time than expected. I estimate I've got over 250 hours invested in various trials, most of which ended in failure. Sure, I could have paid thousands of dollars to have a professional fabricator do the work, but where's the fun in that? Plus, I picked up a few more 21st century fabrication skillz.

In the end it boiled down to this. I selected the following method of fabrication:

  1. Make a scale model of the aeroshell. This is known as a plug.
  2. Using the plug, make a fiberglass mold that's a negative image of the model.
  3. Using the mold, form the various layers of carbon fiber and other materials to create the actual part that will fly on the HAPP.

So it's going to be plug, mold, and part. Ready? Here's the plug...

Plugs are useful because it's often easier to shape or sculpt a model of the part rather then try to directly create a negative-image mold. The plug can be made out of a variety of materials, such as styrofoam, wood, or clay. I know, because I tried them all!

The final plug I used consisted of a wood skeleton covered with automotive styling clay. The wood pieces were cross-sections of the desired shape. I cut the main outer radius of the cross-sections using a router mounted to a giant protractor.

In this photo you can see the outline of the entire aeroshell  - it's a scale model of the Apollo command module from the 1960s. I chose this shape due to it's known stability in supersonic flight regimes. This is important because the HAPP will go supersonic as it descends in free-fall after the balloon bursts at 30Km altitude, only to slow down as it descends to thicker atmosphere.

Giant protractors rule!

As the aeroshell will be fabricated in two pieces - a lower "heat shield" and an upper shell - I separated the heat shield cross-sections from the uppers and mounted them on a base board. This was the first step in shaping the plug.

Cross-sections mounted, I tried to fill in as much of the volume as possible with cheap fiberglass and expanding polyurethane foam. I could have filled it all in with clay, but good clay is quite expensive and also heavy - it probably would have required over 200 pounds of clay had I not used glass and foam.

One little trick I developed was to include a small steel shaft in the center of the plug. This provided an axis about which I could rotate a wooden guide fixture and confirm whether the finished profile is correct. You can see this guide fixture in the following image.

Clockwise from top left:
(1) Laying fiberglass over wood cross-sections.
(2) Wetting out the glass with resin.
(3) Checking glass profile versus target profile.
(4) Filling out the volume with polyurethane.

Next came the surface clay and final shaping. I tried a variety of modeling materials and had several false starts.

Some good, some not so much

After good advice from a friendly professional automotive designer (thanks, Richard!) I settled on Autostyle Clay by Chavant. It can be shaped, machined, and coated. However, before forming the clay, it does need to be warmed up over 100F (40C) or so, which I accomplished using the kitchen oven.

Baking some tasty treats

Bit by bit the clay went on. I pressed and formed the clay into the desired shape, constantly checking and scraping clay with the wooden guide fixture mounted on the central steel shaft. This fixture rotated freely and allowed me to confirm the profile was precisely correct.

Clockwise from top left:
(1) Filling out the profile with clay.
(2) Scraping with the wooden fixture.
(3) Lots of bits after scraping!
(4) Ready for coating.

The last phase of preparing the plug entailed getting a smooth, glassy, and hard epoxy gel coat onto the clay. The gel coat gives a durable surface for creating the fiberglass mold. To do the gel coat right requires multiple coats. I applied three coats with a compressed air spray gun. In between coats, I filled in low spots with automotive body filler and then sanded with a progression of grits from #180 to #2000. Eventually the surface attained a nearly-flawless look and feel which I enhanced by buffing with automotive polishing compound.

To orient yourself, note that you're looking at the bottom of the craft. This would be the heat shield on an Apollo capsule. The plug is approximately 1 meter in diameter.

From the top, each row shows successive layers
of gel coat with filler (reddish color) and the
result after sanding to a fine finish.

At last the plug was ready and I could progress to the mold fabrication phase. This blog post may read like a nice linear story, but it encompasses perhaps 100 hours of reading, trial and error, and tedious model construction. It was fun to do... once.

Next post: Fabricating the mold.